I don’t know that a book intended specifically as an argument against creationism is your best choice. It would be as good or better just to become familiar with mainstream science. That’s what Your Inner Fish is about, for example.
I would strongly recommend Dawkins’s The Ancestor’s Tale, Jonathan Losos’s Improbable Destinies, and for more technical but I think still comprehensible sources, Coyne & Orr’s Speciation and Erwin & Valentine’s The Cambrian Explosion.
Libraries, incidentally, are good for saving money on buying books.