I knew someone had to have started a site just for this;
http://www.whatsthatbook.com/
I looked through, seems like a fair amount of traffic. Many titles appear to have been answered but the original 'asker' didn't mark it as "Answered" yet.
Even if you don't have a book you want to find, maybe you could go over and browse through to see if you can't help someone else out?
-MJ