I heard about an interesting web site from the fellows at The Miniatures Page (also an interesting site!):
This site allows you to catalog the books in your library, and you can see the contents of other people's libraries. Reviews and ratings can be written by the users, and there are groups that you can form or join to discuss aspects of book reading/collecting/writing/etc. There is a wargamer group, with a dozen or so members. I find it interesting to see what the most common books are in the groups!
LibraryThing is free, with a limit of 200 entries. Yearly or lifetime subscriptions are offered for those with larger collections.