I have been buying this guide since it first came out, and I think with this most recent edition the authors have surpassed themselves.
It is especially useful for Londoners like me, who forever see buildings when out and about and to find out what they are. The authors have fully updated it - the Shard comes in for thoughtful short shrift for example, and some of the older descriptions have been revised as well.
The best reason to buy this book is for the amazing photographs, in full colour throughout - surely a real labour of love in a volume as comprehensive as this one.
I think this clearly is THE definitive guide to London architecture currently on the market