This is a really good book of walks, generally around 2 to 4 miles. Andrew Duncan produces easy to follow walking guides that take one to many interesting parts of London, there is a good level of information about what can be seen, and each walk has a map. Although a paperback, the pages are stitched so it isn't going to fall apert if you like to take your walking guide with you.
It will leave you wanting more and therin lies a problem. This book is a compilation of walks from his other books. It has most, but not all, of "Walking London" and a selection from three others.
So my advice would be firstly - definitely get an Andrew Duncan book if you want a walking guide to London. And if you are making a one-off visit, or don't come to London much then do get this book as it really is a very good collection and has 50 walks to try.
BUT if you are a Londoner, or come to London fairly often and if you think you might want to do a lot of walking I would suggest that you don't get this. Instead, get "Walking London" and as you get some done, consider getting "Walking Secret London", "Walking Village London" and "Walking Notorious London" That way you'll end up with more walks to do and no duplication.