"[C]emeteries offer us an accurate reflection of London's social, economic and ethnic history, as well as being a treasury of fascinating stories, courtesy of their permanent residents. - - - Some cemeteries, such as Nunhead, offer spectacular views of the city and their grounds are home to the kind of wildlife and plants you don't see in the course of a normal urban day."
So far, I've only visited Highgate (gorgeous, especially when sunbeams sift through the trees), but this cute little guidebook convinced me there are so many other cemeteries worthy of visiting. Beach gives a short description of each, famous "residents", and practical information. I especially loved the explanations of memorial symbols; they will help decipher the tombstones. Another reviewer spotted a few errors, though, so do a bit of research beforehand and double-check whatever info you need, and you should be good to go.