Yay, I just joined to climb up on the band wagon!
For me, the very worst book I read was the novelization of the game
Baldur's Gate by Philip Athens. Yea, I know, books based on games generally suck but there are exceptions. This is not one of them.

I threw the book down in frustration halfway through because the author started talking about the main character having a voice in his head that was telling him to do things. This same voice had never once been mentioned previously. On top of that, the writing style was dry, the other characters included from the game were silly or killed off without a thought, and the main character was only ten years old but he was fully grown and thought nothing odd about it.
Other books I was dissapointed with were Terry Goodkind's
Naked Empire and
Chainfire. I liked the books earlier in the series up until then, but after Pillars of Creation everything started going downhill.
I was able to suffer through
Naked Empire but the ending really felt tacked on. Goodkind leads the reader on to believe that there is no way for the main character to be saved from poisoning, but at the last second his 'magic gift' is able to spew out a recipe for the others to create an antidote with. It was a very lame way to tie up a loose end. I would have found it easier to swallow if they had discovered the recipe on a piece of paper left in someone's safebox.
Chainfire finally made me give up on the series. The same problems that were presented at the beginning of the novel were still there at the end of it. Nothing really changed throughout the book or was resolved. The only thing it felt like it accomplished was making Goodkind more money.
Oh, and one more series I severely dislike is The Recluse Saga by L.E. Modesitte. Simply because you can read one book and you've pretty much read half the books in series. Just switch out the name of the main character from one book in another and its the same story.
Well, that's the worst from me, although I have to admit I don't think anything can be worse than Baldur's Gate, except the Gor series and that has been mentioned.
