The best book ever writen is "Don Quijote de la Mancha", but is old castilian language, so I'd recomend you some Arturo Pérez Reverte's book, any of them. He joins the Language Academy and he uses the very real and current spanish language. Also you could try Gabriel García Márquez, he won the Nobel Prize and his literature is the best you can find, but he uses many south american words.

Good luck