Assuming you're going "airplane", Rod Machado's "Sport Pilot Handbook" is a pretty good all-around resource. For studying for your rating, I've used the Gleim Sport Pilot Course and have recommended that to students. As mentioned, you can find most everything you need on the FAA website under the training tab. I'd also recommend getting Advisory Circular AC-006B (Aviation Weather) as well as a copy of the AIM/FAR.

Andy Foster
CFI-S/AGI