I've just started cataloguing my library at LibraryThing, something I'd describe as last.fm for books and without a Winamp plugin. It's a slow process: I'm away from my regular library, and the books I've taken on holiday all seem to be fairly strange. My copy of Something Happened (thanks Trav!) either has the ISBN printed wrongly, or the Library of Congress have the wrong ISBN, and I should really use the check digit to see which. I'm proud that it's the oldest hardcover edition listed though. I had to enter the details for my copy of Steppenwolf manually - it wasn't listed at all.
They have a feature (recently written about in Language Log) that lists 'unsuggestions'; books, according to the collected data, that are antithetical to those you are interested in. There are some truly, truly dreadful books out there - the challenge is to find one, a 'seed', that generates a fantastic library. Unfortunately, too few people own Who Moved My Cheese? For Kids for it to generate unsuggestions, but you get the feeling it would work quite well. I found At Knit's End: Meditations for Women Who Knit Too Much which generates a very interesting library, even if it's a little uneven. GEB:EGB, a bunch of Asimov, Clarke, Wells, Pynchon, Vonnegut, Burroughs (Junky), Nietzsche, Phillip K. Dick, Goethe, Camus, Robert A. Heinlein - even Niven's Ringworld. Of course, there's a few Tom Clancys (I want to write Clancies) in there, and C. S. Lewis really doesn't do it for me, but altogether a pretty good mix.
Let us know if you find something worse and thereby better.