But not even Lady Thatcher, whose first volume will be published after the Conservative Party conference in October, is exempt from Civil Service rules: six files is the limit and the borrower must come in person to remove and return them.
Lady Thatcher is not the first Conservative minister to write the story of the Eighties. By the time Kenneth Baker's book appears, also in October, it will be the 13th volume of memoirs from former Thatcher Cabinet colleagues, including Sir Norman Fowler and Lords Gilmour, Tebbit and Lawson.
These publications vary widely in style and quality. As Sir Robert Rhodes James, former Conservative MP and editor of the diaries of Sir Henry (Chips) Channon, put it: 'In his diary Alan Clark admits to rolling up drunk for a ministerial speech. We are unlikely to get any of that sort of thing from Lady Thatcher. It is a very specialised field with remarkably few classics. Rab Butler, Duff Cooper and Churchill stand by themselves'.
Nevertheless the financial rewards for top names are huge, even if the sales are insignificant. Lady Thatcher will earn a reputed pounds 3m, negotiated with HarperCollins by her American agent, Marvin Josephson. Lord Lawson is now believed to have been paid close to pounds 600,000 by Transworld for The View From Number 11, reported to have sold some 26,000 copies. Lord Tebbit's Upwardly Mobile was bought for pounds 200,000 (with pounds 150,000 from the Mail on Sunday for serialisation) and has sold 50,000. Alan Clark's diaries, the most spicy political read for years, earned him pounds 150,000 from Weidenfeld.
The degree of success is determined partly by the quality of writing - Lord Healey and Lord Jenkins scored particularly well on this count, and Mr Baker is expected to turn in a polished volume. Douglas Hurd's diary, which is unlikely to be published for some years, is one of the most eagerly awaited works. Publishing earnings tend to divide into two categories: those up to about pounds 25,000 for the average ex-minister and those counted in telephone numbers for the hottest properties with prospects of serialisation and TV spin-offs.
Membership of the Cabinet is not, however, a guarantee of success. Lord Parkinson's Right at the Centre sold only about 7,500; Sir Norman Fowler's Ministers Decide sold fewer than 3,000 copies.
But prime ministerial memoirs have been lucrative since the early part of the century. Lloyd George was accused of profiting from the First World War through his, and Churchill reputedly became a millionaire from the sales of his writings. Anthony Eden was paid an estimated pounds 100,000 by the Times for his memoirs - a very large amount in 1958. In fact all prime ministers since the Second World War, except Sir Edward Heath, have written memoirs of some description.
The recent publishing expansion has been in the number of memoirs from ministers below the level of chancellor or foreign secretary, even encompassing advisers such as Sir Bernard Ingham, Lady Thatcher's press secretary.
Michael Sissons, the agent who has dealt with many top politicians, believes the recent heyday of the political memoir is drawing to a close. He said: 'Ministers can't just expect to take a big advance and write something anodyne. The books have to have insight and humanity.'
Nevertheless Lord Howe, the former foreign secretary and chancellor, is writing even now, and Norman Lamont is thought to be aiming to produce a potentially explosive book for autumn 1994.
Both memoirs and diaries increase the pool of information available in a notoriously secretive society. But Arthur Seldon, founding director of the Institute of Contemporary History, says that many memoirs disappoint because they do not use ministerial papers, and he rates them well down the list of historical sources. Some take short cuts and are based largely on memory - one journalist recalls going to interview Lord Wilson as he wrote his memoirs, only to find the former prime minister surrounded by yellowing cuttings from the Daily Mirror. Others, he says, devote their energy to paying off old scores, are overly influenced by hindsight or are just too discreet.
Diaries can be of more value, although they too suffer from hindsight when authors get behind and catch up with their writing later. Diary-keeping is, in Lady Castle's words, 'self-imposed slavery'. 'It is a dreadful tyranny to come home at the weekend with three boxes, finish them and then sit down and do your diaries,' she said.
After publishing hers, Lady Castle discovered that the worst thing a writer could do was not to assassinate a colleague's character but to ignore them altogether. 'Curiously,' she said, 'a number of people about whom I thought I was extremely frank told me that I had been very fair. I found that the only colleagues who resented my diaries were the ones who were not mentioned.'
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