Riso was, like, "The man" to me when I first started reading about Enneagram. His stuff was pretty foundational to me, especially the cycles of integration and disintegration; it was a nice blend of the spiritual with the secular. There was a lot of what I would deem "crap" floating around about the enneagram before then; I think he seriously systemized the theory and put out the first very detailed, cohesive descriptions of how the types worked, along with wings and variants. I started taking it seriously because of his work.
He put out a number of enneagram texts; I know I own 2-3 of the major ones.
It's a shame he has passed on. he was younger than my father, who is still around.
From a distance, he actually held his age pretty well:
EDIT: As a complete side note, no one ever told me that Russ Hudson was cute.