If you have just started or are about to start flying, I recommend the Airplane Flying Handbook and the Pilot's Handbook of Aeronautical Knowledge, both FAA Publications. If you can get an old copy of the Flight Training Handbook (it was replaced by the Airplane Flying Handbook) it is worth looking for.
If you like color pictures, the Jeppesen Private Pilot text book is pretty good, but rather costly. I bought it and read it before starting ground school for private and it was nice having already read most of it beforehand.