I think it's a solid idea but one with no real meaning as, in another thread, some people see the stars as a way to rate a story in solely technical terms (is grammar good? punctuation? structure?), some in purely emotional terms (did I like it? Do I like the author?), and others try to use both, or just go with an abstract overall "feel"
Also the fact that ratings apply to an entire book, which may or may not be actively growing, seems a little odd - ratings for individual chapters with an overall aggregate score might mean something more ... but would also probably be more effort than at least 90% of the users of the site are willing to put forth...