This book is an extensive explanation of the origins of the concept of Buddha Nature.

Buddha nature begins with a short sutra of Mahayana Buddhism that revolutionized the understanding of Buddhism.

The concept of buddha nature (which is not just confined to Buddhist so a translation of "essential nature" would be a better translation.

Buddha nature is an essential nature that is possessed by all sentient beings and it is the innate ability to reach enlightenment. We don't usually realize this essential nature because it is covered up or obscured by our disturbing emotions and incorrect beliefs such as the belief that we are a real solid being.

Zimmerman's books is massive of over 400 pages tracing this concept of buddha nature across several countries and several different Buddhist linages and several times.