Since Richard Crossman first spilled the beans on his Cabinet colleagues back in the 1960s, political diaries have become a rich resource for historians and a potent new weapon in the internal warfare that is modern democratic government.

For scholars and interested observers, subsequent publication of the contemporaneous observations of policymakers offers a chance to reconsider the context of long-familiar political events. For the diarists, they serve a much more important purpose. Like unexploded bombs, timed to detonate after their writers have left office, diaries can excoriate critics, punish enemies and deride turncoats, while all the time offering ringing self-validation of every decision made by the author in office.

In the US the revealing political diary has played a smaller role in modern political history than it has in Britain. Lyndon B Johnson famously taped all his White House conversations – and some of his own observational monologues – for later