Anyone who has read Alan Watkins's political commentary will be familiar with his distinctive combination of freewheeling gossip and constitutional pedantry. This book displays the same qualities between hard covers. It offers a comprehensive account of how successive British party leaders have been chosen or emerged, from Bonar Law in 1922 to William Hague in 1997. All the plum moments of the past three-quarters of a century are here, full of juicy personalities and bitter disappointments - Curzon passed over in 1923, Butler in 1957 and 1963, Heath ousted by Mrs Thatcher in 1975, Healey defeated by Foot in 1980, Heseltine by Major in 1990. The format allows Watkins to rehearse again the high constitutional controversies - George V's questionable pressure on Ramsay Macdonald to form the National Government in 1931; Macmillan's manipulation of the Tory succession in favour of Lord Home in 1963 - as well as several murky passages of infighting in the People's Party, from Macdonald's messy emergence as leader after 1918 to Tony Blair's gazumping of Gordon Brown in 1994.

Some of these episodes of internecine strife - Gaitskell against the unilateralists around 1960, the Bennite ferment in 1979-83 - are explored at greater length than their strict relevance would justify. But they are all good stories, authoritatively reconstructed.

This book displays Watkins the historian, not the woolgathering weekly journalist. His method is to meticulously draw together all the existing accounts from memoirs, diaries and biographies, sift them, point up conflicts and errors, and deliver a judicious verdict.

Such is the volume of instant history now that even an event so recent as Mrs Thatcher's defenestration by her own Cabinet is as fully documented from every angle as the manoeuvrings of 1931 or 1963. Watkins's version of this drama is distilled into an account much less chatty than his 1991 book on the same subject, A Conservative Coup. Thankfully, he drops his habit of projecting the current titles of retired politicians back on to their active careers; and he only once calls politics "a rough old trade".

His theme is supposed to be the withering of the royal prerogative. In fact - as Watkins shows - the Palace has long bent over backwards to avoid any appearance of interfering in politics. It has wanted only to be told clearly by the parties whom to send for. Macmillan pretended to observe the prerogative while in practice subverting it; and the new procedure for electing Tory leaders was specifically designed to end it. There has been no case this century of a monarch refusing to grant the Prime Minister a dissolution, nor is there ever likely to be.

This is a useful record, but it is a trifle indigestible: a pudding composed entirely of plums, with little contextual sponge between them. It is all about individuals at moments of high drama. There is little of parties, and less of policies. In his column, Watkins regularly mocks Tony Benn for wanting to talk about "the ishoos"; but politics without issues is just a soap opera - from which the public is becoming increasingly alienated.