Well most people that know me, know that Geophagus are among my favorite fish. I even made one of the fish species, Geophagus altifrons, the logo for our website. One of the only books on Geophagus & their relatives came out in 2000, South American Eartheaters by Thomas Weidner. This 235 page book is the most comprehensive guide to eartheaters printed in English thus far.
While the book is outdated, & out of print now, it certainly is still a useful guide to how to the history, the maintenance of keeping & breeding eartheaters. The layout of this book takes you through the history, the habitat, the aquarium, maintenance practices, breeding behavior & individual species profiles of all known eartheaters as of the printing of the book.
Perhaps if some of the overseas books on South American cichlids are printed into English, those will showcase some of the newer eartheater species, until then, this book is the best guide to these relatively peaceful cichlid sub-families.