I'm fascinated by the many different tyes of aircraft gun-turrets used during the war. I'm in the process of buying "BRITISH AIRCRAFT ARMAMENT" by Clarke (2 volumes for £50 which is a bargain because Volume 1 on Gun Turrets is extremely scarce). The book is a must because there are no other comprehensive guides to the one-off experiments carried out by Boulton and FN.
However, there are a few good websites around:
http://www.navylib.com/Turrets!!!.htm http://www.eugeneleeslover.com/USNAVY/CHAPTER-23-B.html http://liberatorcrew.com/AircraftGunnery.htm