The answer is to look for books everywhere, including thrift stores, which sometimes turn up treasures. Among the scores I've found at a local thrift store was a first edition of the Korea air war novel,"The Hunters" by James
Salter,unfortunately without a DJ, but still worth $80-100. Other finds incude a copy of "Lost Victories" by Manstein, brand new.I paid less than a $1.00 for each of these books.
A friedn of mine brought me copy of "Mein Kampf" in German Gothic script, copyright 1933, in excellent condition. He said it was being discarded by the local Libary. Not really reading material but still an interesting novelty item.
Armor/Aviation books are harder to find, but used book stores, can have have great finds, I recently found a copy of the "Condor Legion " [Luftwaffe in Spain 1936-1939]and paid for it in trade using books I'd found in thrift stores.
In these poor economic times, it is sometimes nice to get real value for your dollar.