There are hundreds of "must have" books on WWII aviation. Some specifically deal with aircraft, others are histories of various types and subjects.
My collection touches on the aircraft of many nations, but the bulk are related to American aircraft. Of those, I will recommend several. Some are out of print, but can be found through online used book services like Alibris.com, and sometimes even Amazon. I'll provide only a few, lest the list become excessive. Here's 20 (23 if you get all volumes) books that belong in any WWII aviation enthusiasts library.
General technical and factual volumes:
1. America's Hundred Thousand by Francis Dean (Schiffer Books)
2. The American Fighter by Angelucci and Bowers (Orion Books, out of print)
3. United States Navy Aircraft Since 1911 by Swanborough and Bowers (Naval Institute Press, out of print)
Aircraft Histories:
1. Hellcat by Barrett Tillman (Naval Institute Press)
2. Republic's P-47 Thunderbolt by Warren Bodie (Widewing Publishing, out of print available from Alibris.com...Expensive)
3. The P-38 Lightning by Warren Bodie (Widewing Publications, hardcover out of print, softcover available)
4. The Corsair 1940-1970 by Bruno Pautigny (Histoire & Collections)
5. Supermarine Spitfire Owner's Manual: 1936 on, all marks by Alfred Price (Haynes)
6. Fortress in the Sky by Peter Bowers (Sentry Books; rare and out of print. Ex$pen$ive!!)
Unit Histories:
1. Protect and Avenge, the history of the 49th FG by Ferguson and Pascalis (Schiffer books)
2. 55th Fighter Group Vs the Luftwaffe by John Gray (Specialty Press)
3. The Black Sheep by Bruce Gamble (Presidio Press)
4. Flying Tigers by Dan Ford (Smithsonian)
Individual Pilot Biographies:
1. Tale of a Tiger by R.T. Smith (self published, hardcover. Out of print, available thru Alibris.com)
2. Thunderbolt by Robert Johnson (re-edited by Johnson in 1997, Honoribus Books)
3. Happy Jack's Go Buggy by Jack Ilfrey (Schiffer Books)
General Histories:
1. Clash of Wings by Walter Boyne (Touchstone Books)
2. Mighty Eighth War Manual by Roger Freeman (Cassel and co)
3. Fire in the Sky: The Air War in the South Pacific by Eric Bergerud (Westview Press)
4. Black Cross Red Star (3 volumes) by Christer Bergstrom and Andrey Mikhailov (Pacifica Press, volumes 1 and 2 out of print, see Alibris.com)
My regards,
Widewing