I read a lot, my husband reads a lot, we hope that our future kids will read a lot.
Ergo we will register for books.
We will NOT ask that our guests get us double gifts, because that's what asking for a book instead of a card is, essentially. I went to one shower that did that, and it was very annoying. Like, "Oh, I know you are already getting me a gift, but would you also get me a book, because just a gift doesn't quite cut it."