I bought a great book back when I first began doing deeper dives from Amazon I think. The name of it is "Deep Diving, An advanced guide to physiology, procedures and systems" by Bret Gilliam. Excellent book! Lots of history about Sheck Exley and his dives down past 700 or 800 fsw (I don't remember the exact depths they were reaching) It was DEEEEEP. (until he died doing it)