As I have often noted to anyone who will listen, there is no "imdb of books." There are reasons for this, beyond the silly notion that nobody has thought of the idea. I can think of three explanations:
1) People don't read as many books as they watch movies. This makes sense, since it is both more time consuming and taxing than watching a movie.
2) There is a greater amount of diversity in books than movies. It is much (much) easier to publish a book than it is to finance and shoot a movie, which means that there is a larger set of books that you need to account for.
3) People who do read books will be less likely to rate them, because they are predominantly older and less technologically literate. Your great-aunt might be a voracious reader but not have the computer skills to let the world know what she thought of each book.
These are three pretty big obstacles, and each of them certainly get in the way of creating the "imdb of books." However, it is because of point #1 that such a site would be so popular. Reading a book is a large investment, both time-wise and money wise much larger than a movie. If there were some way to create a list anywhere close to the imdb's top 250, drawing from a large set of independent opinions, it would be highly successful.
Addendum: Ben Casnocha seems think that TV and presumably movies have less variance in opinions than movies. I disagree, I think it's more of a reluctance to invest (a buyer-side issue), based on a similar argument of Tyler Cowen's here.