Bowyer in one volume does a credible survey of air gunners. Its mostly focused on the RAF but does a great job of chronicling the history of air gunners from pre WWI to post WWII. It is highly recommended for anyone interested in bomber warfare.
Chaz Bowyer (1927-2008) was an aviation historian and author. He joined the RAF, aged 16, in 1942 and left it in 1969. Then he turned his hand to his life-long passion for aviation and started writing.