What type of books do you like? If you mention some genres or titles you like in English, people will be able to make better and more relevant recommendations. Also, if you have read any books in Spanish, it would be helpful to know what they are so we can judge what level of vocabulary and other aspects of linguistic complexity you're up for and give you books that will stretch your brain a bit but not be so difficult that you'll give up on them. I don't mean to be unhelpful, I'm just trying to figure out what to recommend to you.

Los libros mas vendidos de la historia los libro mas leidos del mundo y los mas vendidos los mejores libros.

That being said.. Now, I know that Spanish translations of English books are often rife with errors and also that it really is different reading a book in its original source language, but just for your first foray into reading Spanish novels, you might want to try a book you've already read in English, as this will be much easier. People seem to like the Harry Potter books for this, but I can't comment personally on this as I haven't done it myself. Just from what I can gather from your question and that your profile says your Spanish level is 'intermediate' I would say that you might have good luck with books aimed at teens. It's not great literature, but it's something you'll probably be able to understand.

A bit more advanced, Mario Vargas Llosa's books are selling quite well these days, what with his winning the Nobel Peace Prize recently.

And this site (of a Peruvian magazine) lets you download as a PDF the first chapter of some recently released books. This might be a good idea to see if it's at a good level for you and if it's interesting.