Contact: the Story of the Early Birds by Henry S. Villard (Smithsonian Institution Press, 1987). Written in an engaging style, this is probably the best one-stop overview of the pilots and competitions of the period.
A Passion for Wings: Aviation and the Western Imagination 1908-1918 by Robert Wohl (Yale University Press, 1994). An excellent scholarly account of early flight, written by an eminent historian.
The Sky Beyond by Sir Gordon Taylor (Peter Davies, 1936). The book starts with the author's sometimes-harrowing World War I experiences, then recounts his later aerial explorations, particularly for commercial air routes.
Sagittarius Rising by Cecil Lewis (MacMillan, 1970). A superb memoir of World War I and a great coming-of-age book as well.
No Parachute : A Fighter Pilot in World War I by Arthur Stanley Gould Lee (HarperCollins, 1970). Written many years after the war by a noted Royal Air Force pilot, this is a no-holds-barred look at the war. At times very bitter.
The Great War in the Air: Military Aviation from 1909 to 1921.21 by John H. Morrow Jr. (Smithsonian History of Aviation, 1993). The author is perhaps the leading scholar on World War I aviation history, particularly German aviation.
First Air War, 1914-1918 by Lee Kennett (Free Press, 1991). This book is unique in that it documents the non-combat aspects of World War I aviation, such as the use of balloons for observation and transportation.
World War II
Is there any doubt that World War II has been studied and documented more than any other era of aviation history? We are sure that all manner of works about this period will continue to be written long after the last of the combatants is gone.