If you have any interest in Britain's history and institutions, this is a great coffee table book to own. If, for example, you're a law student, and you want to know what the rolls, as kept by the Master of the Rolls, look like, there's a photo here, with a full page description of the office.
If you're British and you aspire to any of these grand old posts, with or without the ermine, this is probably a good book to start plotting your ascent.
I've also read the slightly more recent 'Jubilee' edition (which I gave to my father as a present and is available from amazon.co.uk). Only a few of the entries have changed, largely to reflect the replacement of Conservative politicians by their Labour equivalents.
If you want another analysis of where the power lies in Britain today, but without the photos, read Anthony Sampson's excellent 'Who Runs this Place?'.