I have had good service from Roll Models at
http://www.rollmodels.net/standard/catalog.htm Although selling mostly model kits and supplies, they also sell books from a lot of major publishers, including Classic Aviation, Albatros Publications, Eagle Editions, Hikoki, Luftwaffe im Focus, Osprey, Schiffer, Specialty Press, and others. If you know what you're looking for (they don't have details on the titles), it's worth shopping there.