I can't think of any books I've ever bought that I just thought stunk, but here's a list of books that I picked up, read (or leafed) through once, and haven't picked up again...
Deities & Demigods (meh)
Return to the Temple etc.
GURPS Voodoo
GURPS Faerie
GURPS Spirits
RIFTS
Palladium Fantasy
On a related note, I have been highly disappointed with the quality of the binding in GURPS 4th edition. I haven't had my Basic Set: Characters for a year, and it's already falling apart! The old stuff is much better bound; and really, I could do without the gloss paper and full-color art.
Now, some folks on this discussion might say that I should have considered the quality of the binding before I paid for it. I think this is a legitimate complaint, because their prior quality of construction led me to believe that their new line would be just as well-bound. Before anyone says that I can't compare softbound books (i.e., GURPS High-Tech) to hardbound books (GURPS Basic Set, 4th Edition), I'd like to say that my hard-bound GURPS Basic Set, 3rd Edition, Revised has taken a lot more thumbing-through, and I haven't even broken the spine or torn a page!
Sacrificing quality of books for flash and glamour is not a good choice. If it comes down to my having to buy another Basic Set because my old one fell apart under treatment that my other books have taken for years (quite mild treatment, I daresay), then I'm afraid Steve Jackson might have to do without any more of my money.
TWK