Yer Ganns and yer St Ex's are all very fine - great books, don't get me wrong, but for me there is only one book in contention.
The author is, of course, Richard Bach, and the book at the top is Jonathan Livingston Seagull.
Guys, scoff if you will (as I once did), but please read it first (it'll take you a couple of hours at most). I was instantly converted, as you will be. It is simply the finest description of the thrill and ethos of learning to become an airman that has ever been written.
And most of what else Mr Bach has written is pretty dam good too.
Otherwise try Sir Francis Chichester's story of flying round the world in a Tiger Moth (including a ditching and total self-rebuild), Adolf Galland's autobiography which read alongside Reach for the Sky gives a superb double take on WW2 in the air. Chickenhawk, as mentioned above is the best helo book,and try Alex Henshaw's Sigh for a Merlin for a differrent take on the Spitfire.
Alex, bless him, is still alive as far as I know, and his utterly gorgeous Mew Gull has recently been sold by the Shuttleworth Collection (why, fer chrissakes?) yet still holds every single piston engined record for London - Cape Town and back and every sector in between. He and G-AEXF set that record in the mid thirties and no-one has ever broken it. Boy would I like the chance...