All literary reference books represent a complex balancing act. They are inevitably the result of thousands of decisions about what to include and what to exclude, which together make up an implicit picture, narrative or interpretation. It is unsurprising and indeed healthy that these works often arouse fierce dispute.

Nothing is ever set in stone. Even if the basic facts about the ancient world and the Bible remain the same, the level of knowledge one can reasonably assume in readers will not. Topics ignored five years before and perhaps soon likely to disappear from sight once again may briefly seem very important, leaving editors with the challenge of steering a path between the genuinely significant and the merely modish. They naturally take account of "received wisdom", syllabuses and student needs, but do they also thereby act as authorities, giving their seal of approval to what amounts to a canon?

And how should they cope with popular writers such as Ben Elton and Dan Brown?

Consider how some of these dilemmas have affected one of the landmarks in the field, The Oxford Companion to English Literature (where Elton, but not Brown, has made the cut). First published by Oxford University Press in 1932, it was radically overhauled in 1984 under the editorship of novelist and biographer Margaret Drabble, who reworked it again in 2000. Dinah Birch, professor of English literature at the University of Liverpool, took over responsibility for the recently published seventh edition, which she describes as "the second big modern revision".

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The Companion has grown slightly and now includes about 7,000 individual entries. "Only a few are unaltered," says Birch. "Some were revised, some contracted and some replaced to make room for new material." Gone, for example, are most of the plot summaries of 16th- and 17th-century dramas and the character entries that told readers in which novels Seth Pecksniff and Hetty Sorrel appear. Such information can now, of course, be found almost instantly via the internet.

In terms of additions, Birch says she was keen to "break down the barriers between 'literary' and other kinds of writing, popular and high culture. So you'll find entries on comics, horror, science fiction, children's books, travel writing and television.

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"Anthony Trollope's Dr Thorne sits next to Doctor Who. In fact, looking at the doctors is quite revealing. There's Robert Southey's The Doctor and Christopher Marlowe's Dr Faustus, alongside Dr Doolittle and Dr Jekyll and Mr Hyde. While I have tried to retain the traditional range of the Companion, I have also greatly expanded its coverage in non-traditional areas."

Although the book remains, to some extent, the view from Great Britain, it now reflects "much more multiculturalism and cultural exchange", puts even greater "stress on dissenting and women's voices" and includes fuller coverage of Arab, Australian, North American and "postcolonial" literature.

The tone of the first editor, retired diplomat Sir Paul Harvey, has often required "de-tweeding", says Birch. Yet she admits she "didn't choose to change the basic format of the book". There remain entries where it is "hard to attribute authorship", since they have never been completely replaced but repeatedly revised by a series of different editors and contributors.

There are two major questions that arise from this process. The new Companion may have eliminated some categories of basic factual information, but it is still full of material that is freely available online (in more or less accurate form). So where should the line between inclusion and exclusion be drawn?

Furthermore, editors such as Birch are well aware of the arguments that have challenged canons, many kinds of authority and even the notion of literature itself. Can they genuinely address such concerns while retaining the "basic format" of a book dating back many years?

The jury is still out on these questions, but it is nonetheless interesting to compare how other sorts of reference tool have responded to these dilemmas.

Orlando: Women's Writing in the British Isles, from the Beginnings to the Present, launched by Cambridge University Press in 2006, is currently five and a half million words long and describes itself as "a highly dynamic textbase". The project is based at the Research Institute for Women's Writing at the University of Alberta, with a site at the University of Guelph, and is overseen by Susan Brown, Patricia Clements and Isobel Grundy.

"We always meant it to be electronic," says Grundy, emeritus professor at the University of Alberta's department of English and film studies. "The functionalities provided by computer came up in the earliest, blue-sky phase of planning a history of women's writing."

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The electronic solution offered a number of key advantages. The most obvious was length.

"Some of us were joint authors of a (1990) predecessor project, The Feminist Companion to Literature in English," recalls Grundy. "It covered all the writing traditions in English and had very short entries, but it still got so doorstep-like that we had to drop the carefully prepared descriptive index.

"Of course no solution is ever perfect. There is a strong motivation both to keep texts for reading on screen to manageable length, and not to let the gap in length widen too far between the most obscure and the best-known writers. But still, online, when you think of something that ought to have been in, you never have to find equal cuts elsewhere."

There is also a downside. Books are often revised, but there is nonetheless a moment when a particular edition is finished and the editor can have a well-deserved rest. Online products can in theory be corrected and extended forever.

"We've actually published a paper on the experience of being 'done'," says Grundy, "and the argument is, by and large, that with an electronic text it doesn't exist. Internally we use the term 'publishable', meaning this entry or that process of checking and validating is complete. But the whole won't ever be complete as long as the thing is alive and generating income to keep a team at work."

Even more crucial are the possibilities offered by "interpretive tagging", which allows Orlando's editors, says Grundy, to "build our tag sets with the idea of covering every kind of topic that might need identifying in writing the history of women's lives, experience and texts".

This enables the information about an individual writer's life and work to be searched by time, place, genre and occupation. One can look at all the authors who were nuns or librarians; who wrote agit-prop, anthems or art criticism; who had links with Scarborough or South Africa. The biographies can also be interrogated in multiple further ways. Such options enable kinds of research quite impossible in a book. But they also indirectly help generate alternatives to more "mainstream" perspectives.

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"Levels of knowledge and information about women writers are still, nearly 20 years after the Feminist Companion appeared, far lower than levels about men, both in universities and among people in the street," explains Grundy. "And a historical account of anything - the French Revolution, the British Raj in India, you name it - which is centred on women and their texts, almost infallibly provides a remarkable 'new-light-on', re-evaluative kind of reading.

"If you run a chronological search on everything Orlando reports for the period of the French Revolution in the category of national/international politics, and then switch to a search on that category and the category of women's writing, you get a wonderful thumbnail sketch of the interaction of events and opinions. Of course if you run a chronological search on witchcraft or suffrage, you would always expect a large women's-interest component, whereas in Orlando if you run one on poetry or the novel (or on Parliament or the BBC) you will get a far higher proportion of material about women than probably from any other source. And we do feel that most sources contain more on men, so it's healthy to have one that contains more on women."

But although Orlando is clearly designed to open up different perspectives, Grundy says that she and her colleagues also "try to resist the emergence of a counter-canon, while recognising that we are probably powerless to prevent it, since a canon is such a convenient thing for teaching purposes and so comforting in general because it enables readers to feel they have a grasp on the outlines of the subject.

"One advantage of the encoding or tagging system is that it does something to ensure representation for the obscure. If you search for writing using non-standard English or set in the Highlands of Scotland, the results will tend to include texts outside any imaginable canon - any that have had the relevant tag attached."

Some of the other options for today's literary reference books are well illustrated in the contrast between two new titles. Open Wiley-Blackwell's new Companion to Twentieth-Century American Fiction and everything seems pretty much present and correct. Editor David Seed, professor of American literature at the University of Liverpool, has brought together a team to contribute, for example, eight pages on Philip Roth, nine pages on Norman Mailer, ten pages on Edith Wharton and 11 pages on Ernest Hemingway. The book also includes a long opening section on "genres, traditions and subject areas", from modern Gothic and "trash fiction" to the western.

The basic structure and rationale of Seed's Companion seem pretty familiar. He stresses that he has included "a number of important contemporary writers who exemplified the surge in different-ethic American fiction" and that "an editor has to keep a flexible, open-ended attitude towards the canon, otherwise you risk recycling an anachronistic tradition".

Yet despite these points, a new book covering some of the same territory could hardly be more radically different. This is A New Literary History of America, edited by Greil Marcus and Werner Sollors (published by Harvard University Press).

It was always intended, says Sollors, who is the Henry B. and Anne M. Cabot professor of English literature at Harvard, to be "not an encyclopaedia, but a provocation". He and his co-editor wanted to create something "startling and entertaining" that was neither "a pre-exam tool" nor "a substitute for approaching the books themselves with fresh curiosity".

"The book does end up giving an account of America from the naming of the continent to the election of Obama," claims Sollors, "but it gives voice to genteel, realist, modernist and multicultural artists, to Puritans, Indians and slaves, to political texts by conservatives and radicals. Reading the book one hears the tonal varieties of American speech, for unlike many standard literary histories and reference books ours does not only permit, but encourages, the use of excerpts and full quotations from primary sources."

The New Literary History doesn't bother with basic factual information or any sort of coherent narrative, although it's arranged chronologically, but brings together a series of disconnected, personal (and often very opinionated) essays that not only offer new angles on the big names of US literature but also consider Alcoholics Anonymous, the Book-of-the-Month Club, Citizen Kane, Dr Seuss, skyscrapers and Superman. There is even a section on the porn star Linda Lovelace, as an early exponent of the "victim memoir", a genre that now dominates the bestseller charts on both sides of the Atlantic. All this, for Sollors, represents "a broadening of what has been considered acceptable not just as context for literature (literary histories always had background chapters) but also as 'literary' in its own right".

He and Marcus were partly inspired by D.H. Lawrence's 1923 Studies in Classic American Literature, and aimed to include writers well beyond the charmed circle of academic literature specialists. So they drew up an almost crazily ambitious "wish list of creative writers we really hoped would contribute. Since we were keen to have a lively and contemporary angle on some of the topics that don't seem to change too much, we asked Bob Dylan to write about Walt Whitman."

In addition to approaching the likes of Philip Roth and Art Spiegelman, they even tried to persuade Barack Obama, when he was not quite so famous, to offer his reflections on Abraham Lincoln's second inaugural address. Although they inevitably got many rejection letters, they can hardly be faulted for trying.

Many contributors ignore the most basic tenets of standard reference-book prose. A chapter on boxing and the hard-boiled tradition in fiction, for example, argues that: "America is a hard-boiled nation ... It's worth a few moments to consider how low our country has come before celebrating and pontificating on the beauty of the language that holds our world the way a bright and shiny stainless-steel garbage can houses maggots and rats."

The book is also unusual in that it embraces the fact that it is unavoidably a snapshot from a particular moment. When they were completing it, recalls Sollors, "the flooding of New Orleans seemed like the most recent moment when things changed". So it originally ended with the editors' reflections on Hurricane Katrina, when "preparations made by the Mayor and the Governor were swept away as they pleaded for help from the national Government, representatives of which either claimed that everything was under control or that they could not confirm what the world was watching on the news ... The city was left to drown."

By the time the proofs were ready, the George W. Bush era was over and Obama had been elected. The New History's editors therefore decided to commission the artist Kara Walker to produce a series of images in response.

"It seemed significant," explains Sollors. "New Orleans was already fading and Obama had to be taken into account. Something had ended, something new had started, but we were not quite sure what - so the book ends on a question mark."

This decision has drawn criticism for its political bias, but it also means that the book will always remain frozen in a brief moment of transition, when America's new President was inspiring a mood of hope but had not yet been put to the test.

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When Sollors spoke to Times ÖйúAƬ, he was on his way to a launch party at the American Ambassador's residence in Paris. He hopes that the New History "may be used as a tool of American cultural diplomacy - not a bad thing after the Bush years". In this way, too, it has ambitions far beyond those usual in a literary reference book.