good book:
The Paphiopedilum Growers Manual by Lance Birk. <---The best of the best!!! If I would have to choose ONLY one, thi swould be my choice!
Tropical Slipper Orchids: Paphiopedilum & Phragmipedium Species & Hybrids by Koopowitz. <---Very good with a lot of photos and information about hybridization.
as per Calcium and Magnesium, not all ofthem like it: Check this website
slipperorchids.info lot's of usefull information here!