The Fiction-L discussion list came up with these social networking sites that are useful for readers advisory — which means for finding books that might interest you, and for sharing books and the enjoyment of reading them with others. Which are your favorites? I'm a big GoodReads fan, personally.... For more, see the Social Networking RA sites page!
Reader’s Maps connect fiction and nonfiction in the Readers Advisory. Using themes other than "writes like X" or "takes place in Y", a map can take a reader through some history and travel, romance and memoir to fully experience a theme.
Readers Maps at Topeka Shawnee Public Library
Bayeux Tapestry includes history,art, and fiction about the Norman Conquest
To Say Nothing of the Dog, based on the Connie Willis SF title offers "Unreliable Narrarators" and "Time Travel" as accompaniments
The Oldest Profession is everything you ever wanted to know about prostitution, including Moll Flanders, and so forth
Staying in Afghanistan, From India to the USA from Nancy Pearl, a handout from PLA 2006
Reading Maps are also listed at Oak Park Public Library
Seattle Public Library is using LibraryThing and is has some great tagging features!