John Grant Catherman was my first cousin, 4 times removed. His father, David Linn Catherman was the brother of my gr gr gr grandmother, Barbara "Margaret" Catherman who married William Geddes.
I'd be happy to exchange information about his family, though I don't have anything more about John himself.
Paul Catherman, author of the Catherman volume you mentioned, was my third cousin, once removed. The book is an incredible resource and is available in quite a few libraries. Unfortunately, an updated "final" edition Paul was working on at the time of his death a few years ago has not been published.