Blindside, I would recommend a general book on the theory of flight to start with, as a gentle introduction, and then Trevor Thom volume 4. As to the former, I have no hesitation in recommending 'Flight without Formulae' by A C Kermode. I picked up my copy for 10p at a summer fair. It was the first edition, printed in 1940, when quite a lot of folk suddenely became interested in aeroplanes!!! I was staggered to find, on a trip to Transair, that it is still in print! It is easy to understand, and a great read - quite superb. By the end, you'll know a lot more about flight theory than most PPLs. I guess it will cost you more than 10p, and, sorry to say - since it has been revised - that it probably won't have the plates in the middle of "The Pilot's Cockpit" (Short Empire Flying Boat), "Upside Down Flight" (The Hawker Hurricane), "An Airship" (The Graf Zeppelin), and "It's the Same the Whole World Over" !!!(an Me109 with Handley Page automatic wing tip slots, variable camber flaps, elevators with horn balance, .....).