Country Walks Around London by Geoff Garvey and Leigh Hatts
I spent two years living in London while studying at London Business School. Stuck out in the ‘burbs of Metroland (Willesden Green) it was good to escape to the countryside for the occasional walk. One of the things I found remarkable about London was that, despite the number of people living in the Greater London area, it was still possible to get away from the crowds while remaining within the M25 boundary. This book helped guide me to some excellent walks, all serviced (at the time at least) by public transport – useful since I didn’t have a car.
Now sadly out of print, but still available used from Amazon resellers (and presumably other second-hand book stores) I can strongly recommend this book. While the maps are a little basic, the descriptions are (or at least were) excellent; the authors have clearly walked the routes, and they have chosen some good ones. Just remember that the book was published in 1998 so conditions on the ground may have changed since then.