Suggest "Aerodynamics for Naval Aviators" as a very good intermediate treatment. Not so much maths that it is accessable only to engineers and a lot more than the usual cartoons etc in PPL materials.
Not sure where it would be available (I got mine when I worked for the Navy from a retired ensign I knew) Try Amazon and then as I believe it is a publication of the U.S. N. start with the Navy Air Systems COmmand or the Navy Test Pilot School or wade into the U.S. Government Printing Office (the U.S. counterpart to HMSO).
Good Luck