These four books will give you a world of study material:
Rotorcraft Flying Handbook - FAA
- read this first, simple concepts and descriptions of manuvers
Private Pilot's Handbook - Ron Machado
- the best and most interesting overview of the world of aviation - don't be put off by a seeming airplane bias, 90% of the book is for all pilots.
Cyclic & Collective - Shawn Coyle
- goes in-depth through the world of helicopter flight. It might be a bit deep at first...
Principles of Helicopter Flight - W. J. Wagtendonk
- fairly technical, cut and dried, but if you are going through to CFI, you'll need the info in this book.