http://www.supsalv.org/pdf/DiveMan_rev6.pdf
It's got a pretty good chunk on the history of diving and the development of diving gear (1st Closed-Circuit system was developed in 1879), diving physiology and physics, and more than enough formulas and equations to make my brain hurt.
...granted, it doesn't take much math to make my brain hurt, but I'm sure there's someone somewhere who can use it to DIY a dive computer.
