Named a Choice Outstanding Academic Book of 1997, and featuring a foreword by the Dalai Lama, a lavish study of a Tibetan Buddhist initiation ritual includes diagrams, photographs, and visual representations of the mandala principle. Reprint.
This book is definitely the closest you're gonna get to a clear explanation of the Kalachakra mandala in English. Even the author admits that he doesn't fully understand it and it's always mildly incomprehensible at best, but there are a lot of charts and graphs and he really gives it his best. Discussion of other mandalas, mostly Tibetan, as well.