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Try to get "Freedive" by Terry Maas/David Sipperly.(ISBN # 0-9644966-1-5)
Pretty comprehensive, but easy to understand book on all the basics and various applications of freediving, even has a chapter dedicated to UW-photography. I'm sure, you'll find this book a very helpful source of info.
it just makes it harder to convince the wife that I need to spend $50 so I can "Hold my breath longer"