Flight training is expensive. But when it comes to study material, you can save yourself a lot of money by getting your information straight from the source. All of these books are available FREE from the FAA.
This book is full of great information to get you on your way to learning how to fly.
Also know as the P-HAK, this handbook goes more in detail about the things you need to know to fly an airplane.
This hand book is a great resource to use when you start Instrument training.