"Flight Theory For Pilots" by Charles Edward Dole is amazing. I am reading it now- very easy to understand (assuming you have a basic physics/math background... ie) highschool).
I also read "The Advanced Pilots Flight Manual" by Kershner, "Stick and Rudder," and "See How It Flies" (it is an online book).
All the above books are good, and I would recommend each.
Now, the FLight Theory for Pilots book deals a lot with power/thrust required curves and stuff like that. If you have never heard of or seen these curves before, it can be confusing. He also uses a lot of formulas, but he shows how he derived them, and this is very basic.