Did you know that each resort has a book swap shelf in the lobby? You can leave the books you have read and take a new one, as more people start participating, the collection will grow.
If you have finished all the books you brought down with you and urgently need some more beach reads, the Needful Things store on Avenida Coba in Downtown Cancún and Alma Libre in Puerto Morelos both have a good selection of secondhand titles.