I tend to stop reading books pretty early if I can tell they're going to be a long, pointless slog. I stopped reading Wizard's First Rule (recommended highly to me by a friend of mine who mostly reads those and the Dragonlance books) when I noticed that it was just Ayn Rand in Middle-Earth, and I almost read the first Wheel of Time book when I noticed that it was about virtually nothing. I'm also still in the middle of Atlas Shrugged, and I've got The Fountainhead sitting on my shelf there. Might make time for those someday soon.
One dull book that I had to read cover-to-cover (for a school assignment) was Driver's Ed, by Caroline B. Cooney. The ENTIRE BOOK was about a bunch of teenagers moping because some woman crashed her car and died. If you cut out anything that wasn't plot, the book would be about 20 pages long. Most tedious thing I ever read all the way through. Guh.
Redwork by Michael Bedard was also pretty awful. It was about some kid that lived in a flat above an old man who I think was an alchemist, but it was so boring that my CLASS didn't finish it.