A good one is 'Winged Victory' by V.M. Yates, (or Yeats?).
It was written by a WW1 Camel pilot, and far from being Biggles-style escapism is a deeply moving story of a Camel squadron and its exploits in France. It seems to have been written in the spirit of putting down things as truthfully as possible, so it's easy to relate to the fears, excitement, war-weariness and so on that is portrayed, as it all reads so true.
Another excellent book is 'Wings on my sleeve' by Eric Brown (I think). He was a British fleet air arm pilot in WWII who went on to become a test pilot, flying all kinds of weird and wonderful machines, including testing some of the German types just after the war ended. A bunch of great stories, and well written.