THE "Bible" for me was Shep Piane's How to Build Dioramas... That one book was my Grail...
Now that there's this World Wide Web thing, I don't buy reference books anymore... Not to mention that it's pretty rare to find anything nowadays in a book that ain't on the 'net... As long as you kow how to search with the right keywoods, that is... You'll be disappointed if you search under Me-109 interiors" vs Me-109 cockpits...
For entertainment and inspiration though, non-fiction books are still the way for me to get ideas, especially unit histories...