Return of the Nephilim
by Alfonso Caso.
University of Oklahoma: 1970.
Wonderfully illustrated by Miguel Covarrubias, this book is at times a general overview of the confusing and complex Aztec religion and deities, and at times provides insight into some interesting details.
Intended as a popular introduction, this is also a great reference. His economy with words and clear prose make a difficult subject more accessible to us non-experts.