There has been only one book that I wish I never read:
The Girl Door by Jack Ketchum.
I can stomach some really rough stuff, even alot of the "torure porn" books and movies...but this....this is just disgusting, and it really made me stop reading at times. It's loosely based on a true story.
The first review in the link pretty much sums up what I felt about it...and I even get a sick feeling just thinking about it....but I wouldn't read it again.