Air University Press, the publishing arm of the USAF's Air University, has most of its books available in PDF format for free download. As one might expect, the subject matter is mostly American and recent, but some are on-topic for me, including Williamson Murray's Strategy for Defeat: The Luftwaffe, 1933-1945, George K. Williams' Biplanes and Bombsights: British Bombing in World War I, William Edward Fischer's The Development of Military Night Aviation to 1919, and Philip S. Meilinger's The Paths of Heaven: The Evolution of Airpower Theory and Airmen and Air Theory: A Review of the Sources.

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