Some bloody good suggestions there especially with regard to Alibris. Also have a look at Abe for second-hand books.
Adding to some of the Commonwealth air forces books above:
A Thousand Shall Fall by Murray Peden - beautifully written by Canadian pilot flying Stirlings and Fortresses.
Life's Too Short To Cry by Tim Vigors - BoB ace and flying Buffaloes over Singapore.
Down To Earth by Owen Zupp - biography of a 19-year old Hurricane pilot shot down over the beaches of Dunkirk, Turbinlite ops, flying Hurricanes over Dieppe and Mossie intruders during D-Day and beyond.
Not Peace But A Sword and
Torpedo Leader by Patrick Gibbs - legendary antishipping leader flying Beaufort torpedo bombers over the North Sea and out of Malta.
A Gremlin On My Shoulder by Ron Cundy - Australian Kittyhawk ace with 260 Sqn RAF in the desert and flying with the likes of Canadian ace Stocky Edwards.
Night After Night by Max Lambert - New Zealanders in Bomber Command. Brilliant read.
Jack Davenport, Beaufighter Leader by Kristen Alexander - one of the great leaders of 455 Sqn RAAF.
The best American-written book I have read (got a heap still to read) in the past 10 years is
Into The Teeth Of The Tiger by Don Lopez. Flying P-40s in the CBI.
Can recommend at least another 50 books at least from my collection but haven't read them all so...
You could also check out my website below. It might be of some help.

All of the books I've mentioned above, except the Don Lopez one, are reviewed there.
Let us know what you buy.