Donkey, Burro, and Mule books at Browsers Uncommon Books, updated 14June2012
The Mule Companion: A Guide to Understanding the Mule by Cynthia Attar, illustrations by Lori Mirmesdagh, paperback.
Mules I have Known & The Good Ol' Boys by Max Harsha
Asses vs Jackasses by William G. Long, Hardcover. 134 pages.
Travels In A Donkey Trap by Daisy Baker
The Burro Book, This copy has sold.