Chthon by Piers Anthony. Ballantine, 1967. Cover uncredited.
As far as I know this is Piers Anthony's first book. It's about a woman who can only love when she's getting hurt. I think he was better served moving to Xanth.
The Cat Who Walks Through Walls by Robert Heinlein. Ace, 1985.
This is the weirdest credit so far: "Cover: "For more information about the cover art please contact Glass Onion Graphics PO box 88; brookfield; CT; 06804"
Inferno by Larry Niven and Jerry Pournelle. Timescape, 1976. cover uncredited.
This is a personal favourite in which a skeptical science fiction writer dies and takes a trip though Dante's nine circles with a very unexpected guide.
Triton by Samuel R. Delany. Bantam, 1976. Cover uncredited.
While hunting in vain for the name of the cover illustrator I instead found this nugget: "The lyrics on pages 171-174 are by Bruce Cockburn from his album Night Vision and are reprinted by permission."