Colin Matthew edited one prime minister's papers that were donated to the nation. William Ewart Gladstone (1809-1898) was four times chancellor of the exchequer and four times prime minister. He was in office in every decade from the 1830s to the 1890s, a span two decades longer than Churchill's, and throughout he was constantly controversial.

Gladstone's moves from Tory to Peelite to Liberal - made at Cabinet level - infuriated many contemporaries but, unlike most 20th-century Cabinet members who have changed parties or founded new ones, he retained his position at the forefront of British politics. He represented Victorian finance at its most orthodox but in his proposals for Irish Home Rule offered a solution that significant elements of the predominantly English Establishment could not stomach. From the perspective of the late 20th century, Home Rule looks an eminently sensible and even modest way of pacifying Ireland constitutionally.

Gladstone documented his tempestuous career thoroughly. He kept school-friends' letters and his own juvenilia. His huge archive was divided after his death into three parts. The political and literary parts are in the British Library, the "minor" correspondence, family and estate papers are in St Deiniol's Library, Hawarden, and his diaries and some accompanying papers were given by his sons to the Archbishop of Canterbury, ex officio, and consequently are in Lambeth Palace Library. Gladstone began to write a diary while a schoolboy at Eton (the earliest surviving entry is that of July 16 1825) and he kept it daily until May 23 1894, and then episodically until the final entry, written on his birthday on December 29 1896. There are therefore about 25,200 entries - a long diary by any standards and unmatched for length among politicians.

It is the diary of a politician, but it is not a political diary in the modern sense. It was written more as a record of events and activities than of political comment or observation. Religion, and private rather than public religion, is its backbone. It is a skeleton clad in very lean flesh. It is striking that he kept it going. Very few prime ministers have the self-awareness and self-criticism to keep a diary while in office; consistent diary-keeping is usually a mark of the second or third-ranker.

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For the Gladstone family, it was also a skeleton in the cupboard, for it contained much detail about the temptations Gladstone underwent in his "rescue work" with prostitutes and the self-flagellation he inflicted on himself to purge his guilt (when those entries in the diaries were printed, Oxford University Press had to design a special font to represent Gladstone's sign for a whip). John Morley used the diary discreetly for his 1903 biography of Gladstone, but perhaps unsurprisingly, given the times, steered clear of such passages. Morley made no reference to Gladstone's qualified declaration of marital fidelity, written and sealed just before he wrote the final entry of the diary. Gladstone was aware of the perplexities his diaries would cause his family and others, but he had not destroyed them or mutilated them in the manner of a number of late-Victorians. His sons' donation to the archbishop of the diaries and of various ancillary papers about "rescue work"was a neat solution.

In 1954 Philip Magnus's Gladstone for the first time gave a proper place to the importance of "rescue work" in his private life; but Magnus had not had access to the diary and underestimated the extent to which danger and temptation were mixed with charity in Gladstone's activities. Neither did he mention Gladstone's remarkable and longstanding relationship with Laura Thistlethwayte, a former courtesan who had become an evangelical preacher. Archbishop Fisher decided the time had come for publication. Magnus was thought of as a possible editor, but was blackballed by Tilney Bassett, the Gladstone family archivist, and M. R. D. Foot was chosen. In 1968, Oxford University Press published the first two volumes.

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In 1970, I succeeded Foot as editor and expanded the edition's base to include, in addition to the Cabinet minutes already planned, the extensive memoranda Gladstone kept pari passu with his daily diary and also a selection of the correspondence, the recording of the writing of which was such a focus of the diaries' entries. The 13 volumes thus contain a large number of memoranda, the minutes of about 550 Cabinets Gladstone chaired, and nearly 3,000 letters written while prime minister.

The sheer scale of Gladstone's diary is intimidating: he mentions more than 20,500 individuals and records reading more than 17,500 books by more than 5,000 authors, and thousands of periodical articles. He was the first prime minister to attempt a systematic record of Cabinet business and, since there was no cabinet secretary, had to take notes while chairing the meetings, so it is hardly surprising the results are often hard to read.

The introductions to the volumes were a challenge, being written rather quickly every four years or so whenever the text of each volume was nearly ready. This meant there were nearly 25 years between my first and last effort, while the interpretatation they offered had to be consistent.

Gladstone's defeat in the general election of 1874 and subsequent resignation as leader of the Liberal Party was a natural caesura in my interpretation of his public life.

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The Gladstone Diaries is probably one of the last large editions prepared for paper publication only. The rapid expansion of electronic publications and the declining purchasing power of libraries point to such undertakings being in future both print and electronic at least. Undoubtedly, computerised text makes for far easier and neater editing. But in the 1960s such things seemed worlds away, and changing halfway through would have been expensive and time-consuming. The availability of library catalogues on computer cut by about a half our research time on Gladstone's reading in the final volumes. Future editors will spend less time hunting for missing bits of typescript but will have to raise more initial capital. Ironically, technological progress means large editions will be more expensive though, probably, more expeditious to produce. Readers will want books but they will get CD-ROMs. Gladstone stands at the end of a publishing as well as a political epoch.

Colin Matthew is professor of modern history, University of Oxford. He is now editing the Dictionary of National Biography.

The Gladstone Diaries with Cabinet Minutes and Prime-Ministerial Correspondence (Clarendon Press): Volume 12, 1887-1891 (Pounds 65) Volume 13, 1892-1896 (Pounds 60), Volume 14, Index (Pounds 60).