Literature Map is a fun site that I just found out about from the excellent blog, iLearn Technology. This is a great site to find out similar authors by typing in an authors name and watching as the cloud forms.
Below is an example of a Literature Map of Stieg Larsson...
Once the map is created, a user can click on another author to form another map from that author.
Give Literature Map a try by clicking here.
For other fun sites w/ books check out Book Lamp or Book Seer.