A symbolic representation of the elements of the universe, the mandala is a mysterious and often misunderstood sacred image and ritual tool in Tantric Buddhism. The Mandala provides a thorough study, with sensational color photographs, of the different aspects of this traditional form of sacred art.
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.