Today N. K. Jemisin's The Killing Moon came around on the guitar, interlibrary-loaned. It's paperback, and remains so; the owning library didn't stiffen the cover. (I'm not sure how common or unusual this may be. My library tends to stiffen paperbacks, but it does cost money.)
The back half of the book has two large wrinkles in it, in the pattern of an index and middle finger - significantly larger than mine. A man's hand, I think. Someone who holds the book in two hands, the right side held so firmly that the pages bend permanently. Maybe he felt strongly about the book. Maybe he just holds books tightly.
Or possibly someone dropped it in the bathtub, and it's wrinkly from that. I don't think so, though. I don't see water damage, just a lot of bending. And it is remarkably like a handprint.