Yes, Prime Minister (by Antony Jay and Jonathan Lynn)

(Amazon UK link)

'Yes, Prime Minister' is a diary-style book by Jonathan Lynn and Anthony Jay. Slightly unusually, it is based on the brilliant BBC TV series of the same name, rather than the TV series being based on a book. Hence the cover photo is of the three main characters in the television show. 

This book contains the supposed diaries of Jim Hacker in his role as British Prime Minister, interspersed with letters and memoirs. There are brilliant insights into the workings of the Civil Service and the government, with plenty of humour, although perhaps it gets rather too close to the truth at times...

I'd read this book before, some years ago, as well as having seen most of the television series, but thoroughly enjoyed a re-read.

Definitely recommended.