I'm trying to expand the range of universal/generic systems I'm acquainted with, and took a look at Unisystem. I had a very superficial run-in with it in one of its licensed-franchise forms some years ago, and would now like to take a look at it in its pure, setting-agnostic form. But when searching publications related to the system, I am having trouble finding a book that would be the equivalent of a system core (similar to the role of Cypher System Rulebook of Cypher, the EABA 2.0 book of EABA, the Basic Set of GURPS, and the like).
What book should I look towards for acquainting myself with the system in its most basic, generic form?