When looking for reference material on aeronautical subjects, I have two main sources: Amazon and Alibris. For those who don't know about Amazon, it's a major internet-based book-seller that should be checked first when searching for any technical book - their inventory is vast. Alibris is lesser-known. They tie together an international network of libraries and independent bookstores that have used books to sell. I have now purchased half a dozen books from Alibris and every one has been in fantastic condition (keep an eye out for bargains - and overpriced books, too).
I know that the following very useful books can be found on Alibris:
Perkins & Hage, Airplane Performance, Stability, & Control
Abbott & Von Doenhoff, Theory of Wing Sections
Marks' Standard Handbook for Mechanical Engineering
Roark, Formulas for Stress and Strain
Bingelis, Firewall Forward
A subtle advantage of Alibris is that it's possible to find previous editions of things that have since been updated. I am told that this is important for books such as Roark's (there are some purists who believe the later editions are too theoretical).
There are, though, certain texts in aeronautics that are both highly valuable, and very hard to find. Among these are the texts that have been independently published by their authors, therefore don't show up through the regular search pathways. To make it easier for members to find these books, here's a list of sources for the rare books that every aeronautical engineer must have. A company named DARcorp has many rare books available (including some in this list). www.darcorp.com I will do my best to add new links whenever I find them. Thanks to BVF, terning, Wittenborn, and edbgtr for their input. Anyone with additions can e-mail me (see my "profile").
Analysis & Design of Flight Vehicle Structures, by E.F. Bruhn