The US Air Force Historical Studies Office has put up several dozen monographs on the history of the USAF and its predecessors, PDFs available for free download. It seems to be more narrowly focused than the similar effort by Air University Press, as only a few titles look like they might discuss the RAF in any detail: D-Day 1944: Air Power Over the Normandy Beaches and Beyond by Richard P. Hallion (1994), Preemptive Defense: Allied Air Power Versus Hitler's V-Weapons, 1943-1945 by Adam L. Gruen (1998) and, rather oddly, Coningham: A Biography of Air Marshal Sir Arthur Coningham by Vincent Orange (1992). The most interesting, however, given a recent post here, is The Command of the Air by Giulio Douhet (1927, translated 1942). Via WWII mailing list.

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