Having lived in London for a decade and thinking I know where most things are, I found this guide really useful in that it lists shops and useful resourses that I wouldn't have found out myself, eg RF Greasby an auction house that holds sales on behalf of the Met Police and London Transport (for bikes, cameras) and advice on how the auctions work; graded white goods - where all the ex-display stock goes (expensive brands too) and lastly a little vintage shop In East Dulwich that had been there for years and as a local I didn't even know existed! Lots of shopping maps for different areas too.
This book doesn't have every single bargain spot, but a pretty good selection. With it I have discovered plenty of shops I might not otherwise have done. Some of them were quite eccentric, so I didn't just save money.