The book you are thinking of is Forbidden Arcana.
There isn't a "variant" mystic adept, but there are several new types of magical Talents: Apprentices (partial mages limited to a single school of magic, rather than just Spells/Conjuring), Explorers (only capable of Astral Perception and Projection), and the Aware (only Astral Perception).
None of the new aspects have access to Adept Powers, so they aren't Mystic Adepts.