"The Horse's Mind," by Lucy Rees (Prentice Hall Press 1985).  The author notes (pages 22-24) that when a horse rolls its eye back and you see the white, it doesn't necessarily indicate temper or viciousness but only that he is trying to look behind him. This is just one example of the insights provided by this oustanding book, which is, hands down, the best explanation and analysis of how horses think and behave in respond to stimulus that I have ever read.  The book jacket says, "Lucy Rees runs a riding school and breaking and training center in Wales.  The skills she brings to training 'problem' horses are the result of lifelong experience with horses and her education, which includes years of study of zoology, neurophysiology, and behavioral science.  She has worked with totally wild horses, polo ponies, show jumpers an din the United States each year, with Morgans, Saddlebreds, cow ponies, and rodeo ponies."  I believe this book is out of print now so you might have to work to find it, but it's worth it.