Obviously most of the books that you find in Spain are written in the Spanish language (a no brainer, I know) but you can find lots of English ones too.
Libros is a chain bookstore, much like Barnes & Noble back in the USA, and they always have a selection of books in English.
I was looking through one of the stores and spotted one of my favorite books there. I'm not talking Tom Clancy or Dan Brown or Ernest Hemingway, but John Fante. Who?, you may ask. Go grab yourself a Fante book, if not The Bandini Quartet then just one of the stories like Brotherhood of the Grape and discover a brilliant much forgotten American writer.