Without mentioning the Web X.0 cliché, Reader2 is a site definitely in the mix with elements of folksonomy, social networking, RSSing for people to build collections of books they have read or want to read.
It’s pretty quick and easy to add book titles- the add form allows to enter a title and use an Amazon search- it Ajaxes the results right in the page, copelting the author, ISBN etc. You can add your own series of tags, a descipription of the book, or a review. It has dropdown categories of books that is awfully limited (please make it more extensive or something we can configure, why cannot this be tagged as well?); as well are the “status” you can assign to a book- again, make it more open or extensive.
I’ve been playing with it off and on for a week…