I’m reading a slender volume by Claudio Narano—Ennea-Type Structures: Self Analysis for the Seeker. Naranjo is a Chilean psychiatrist, now in his 80s, who lives in Berkeley. He is a founding interpreter of the enneagram of personality. He was a close friend of Carlos Castaneda, a pioneer of entheogenic studies, and one of three named successors to Fritz Perls, the founder of Gestalt Therapy.
Naranjo learned the enneagram from Oscar Ichazo, the Bolivian-born founder of the Arica School.
The Enneagram Institute website is an excellent starting place to learn enneagram. Here is its historical section.