The U.S. Navy Diving Manual is a very complete reference on diving based on the extensive research and experience of the U.S. Navy. The manual contains major sections covering rebreathers, no-decompression diving, open-circuit SCUBA, surface-supplied diving, the diving environment, Navy dive procedures, physics, physiology and recompression chamber operation. The U.S. Navy Diving Manual underwent some significant updates with the recent sixth revision issued in April 2008. One major change is their decompression procedures and the introduction of a completely new decompression algorithm and table.
Although much of the information is interesting, it may have limited application to your recreational diving. Still, recreational diving has some of its roots in the almost 1000 pages of this document. Best of all, it's available at no charge because it was developed with taxpayer funds. You can download the U.S. Navy Diving Manual, Revision 6A (15OCT2011) as a PDF format document from the Navy's own website. The file is large, 20 MB, so best to right click on the link and "Save..." it to your hard drive rather than open it in the browser.