Warm. Definitely. I've heard stories of cats who hate each other, but they'll all sit on a heated tile floor together. It makes sense for a cat, whose inner temperature is about 102, to seek out warm places to keep the body temperature up. When I'm sick and I have the flu, my cats are over me. All seven of them. And I used to think, "Oh, they want me to feel well." No. They want to stay warm. So, it's possible that your cat is also going for the warmth of the book from your hands. And with a new book, unused, the cold could be shocking.
I notice when Bubba is lying on these books, he rubs his face against the edges. What's he doing? Does that just feel good?