I’ve wondered this too, but have never found anything. Perhaps another avid reader will know of something. I am so tired of reading half a book only to realize I hate the direction the plot is going and I can’t stand the main character’s love interest.

One thing I could say would to go to the book you liked page on Amazon and it will show you related books/what other readers also bought. Really handy feature, I’ve used it many a time and found some real gems.

A knowledgeable book clerk in a book store can recommend other authors and titles you might like.

Pretty sure i saw a service like that some years ago.
I will have a search.

Edit: These are not what i remembered but they may do the trick?
Goodreads, LibraryThing

Amazon will make recommendations based on your purchases. In lieu of that, find a librarian who specializes in reader’s services whose taste you trust.

Goodreads is the closest thing I’ve found to what you’re looking for. I go to a favourite author’s page, click on a random fan of the author who seems to have a taste in books similar to my own, and then lurk the things they’ve read.
It’s not as convenient as last.fm for music, but it’s the best thing for books that I’ve found.

I think Whichbook tries to do this, if anyone else is interested.