I realize that it's best to choose books written in Spanish originally, but I had a great experience (and learned a ton of vocabulary) reading The Golden Compass (or La Brújula Dorada) by Philip Pullman. Personally I find that, while I LOVE GG Marquez, that in Spanish his words and use of language are SO poetic (and gorgeous, love it) that it's not really good reading for learning phrases and words to use in daily life. YMMV!
