Fine, I've pulled the mechanics textbooks out of my bookshelf. The two textbooks by Feynman and Goldstein refer to a center of gravity; I've added them to the article with quotations and page numbers. On the other hand, the following textbooks define the center of mass but not a center of gravity:
Arnold, Mathematical Methods of Classical Mechanics
Marion and Thornton, Classical Dynamics of Particles and Systems
Kleppner and Kolenkow, An Introduction to Mechanics
Jose and Saletan, Classical Dynamics: A Contemporary Approach
I haven't decided what to do with them.