'Flight Without Formulae' by A.C. Kermode. I read it during my GCSE's way before I started PPL and found it to be a good intro.
It covers the basic principles of flight, explained as simply as possible, excluding all mathematical formulae. It's intended to be of use as an introductory text for trainee pilots and students as well as for the general reader.
...and no I'm not on commission
Happy reading,
ATCS