Screwtape Letters by C.S. Lewis, The German Army on the Somme by Jack Sheldon, Weapons and Tactics of the Soviet Army by David Isby, and all kinds of stuff on A6M2a Model 11 and A6M2b Model 21 Zeroes for a "batch build" I'm getting started on this week.
I always have at least two books going at once when I can afford the time.
Warren