If you're not finding that certain scuba diving book at Amazon, head over to Best Publishing. They publish scuba-specific books for hard-to-find topics about technical diving, equipment, dive medicine, history, commercial diving, and many more. Some titles:
The Technical Guide to Gas Blending
The History of American Deep Submersible Operations
Histories and Mysteries: the Shipwrecks of Key Largo
Dive Like a Pro: 101 Ways to Improve Your Scuba Skills and Safety
Adventures with Sharks
On-site Management of Scuba Diving and Boating Emergencies
AIDS Under Pressure
The on-line store is a source for new and used copies ranging in price from $6 to over $100 based on the size and subject. You'll find other publishers' book in their store (such as NOAA guides and Shadow Divers) which makes it difficult to discern their publications from others. Well, that's not entirely true if you browse long enough. Best Publishing's book covers tend to give it away, going from a text-book-like clean and neat to one that would make Robert Crumb proud.