Dr David Hunt’s Huitzilopochtlia is now available to download at the Cactus Explorers website.This is a great resource for the genus, and has a lot of species data, descriptions, etc including distribution maps.
CactiGuide is a useful source of information for many cactus species including Mammillaria
Ralph Martin’s Field Number Database
Rarecactus Field Numbers Database
Plants of the World Online is a link to the latest thinking on taxonomy