One of them is The Big Book of Sumo: History, Practice, Ritual, Fight, by Mina Hall. It's an oversized paperback and combines text, drawings, diagrams and photos. You can view parts of it at Amazon.com, where reviews are positive. Here are a couple of examples of the drawings:
from the back of the book:
In a dirt ring, two giant men perform the age-old ritual, stomping and clapping to alert the gods. Then, in a great burst of energy, they charge forward and grapple each other to determine who will be champion... The Big Book of Sumo is an illustrated guide to Japan's ancient ritual sport that today is winning new fans around the world. Super sumo fan Mina Hall covers all aspects of sumo, including history, traditions, training, stable life, rankings, fighting strategies, and judging. She goes inside the "stables" to show the rigorous, disciplined life of a sumo wrestler. She also gives advice on how you can visit a stable yourself and where to eat and stay near the main sumo arena in Tokyo. Of special interest are a collection of belly-busting "sumo stew" recipes and a revealing look at the Hawaiian power-houses who have helped make sumo a truly international sport. Whimisical line drawings make The Big Book of Sumo the most user friendly sumo guide book available -fun to browse through and filled with amazing facts about the giant wrestlers of Japan.It's a wonderful guide to understanding all aspects of the sport.
The other book he gave me is The Joy of Sumo: A Fan's Notes, by David Benjamin:
This book has more text, fewer and less helpful illustrations, more details about certain things but less general information... In the introduction the author says, "Herein, then, is freedom from expert opinion! I will guide the casual but astute fan through the stages of sumophilia."
from the back of the book:
Just what is sumo? Is it a sport, a religion, an aesthetic feast, living traditional culture, or a battle of titans? The simple answer is Yes! For the first time ever, David Benjamin's The Joy of Sumo strips away the veneer of culture and looks at Japan's national sport through the eyes of a true aficionado. Learn what makes a sumo exciting. Read what makes individual wrestlers tick. Understand why there are no weight divisions. Find out how to remember names. Memorize tactics. But most of all grasp why such a plethora of descriptions of the sport can all be true. Sumo, like any sport, has its heroes, horrors, and hilarity, and The Joy of Sumo has something for everyone. Guaranteed to provide invaluable information for everyone form the novice fan to the expert, The Joy of Sumo also is the first book to do it with the same humor normally associated more with football, basketball, or baseball. In fact, after he's read this book, the sportslover's world will never be quite the same-just broader and more fun!You have to be willing to actually read this one to get anything out of it, which makes it much less useful to me if -for example- I just want to know what just happened in a bout. It's more filled with anecdotes about past wrestlers and answers fewer Why? and What happened? questions.
Together they make a wonderful resource as I try to learn more.