Pretty Pretty Horsies: A History of the Mongol Empire
This book is used in the quest Oh My Papa if the player character fails the Speech checks required to have Papa Khan break the alliance with Caesar's Legion. Instead he will ask you for something that will guide his people when he is gone. If you give the book to him "as a reminder of what his tribe should be like," he will break the alliance.
The book is found in a footlocker in Caesar's tent if you are not taking the quest. If you are, then it is found at the 188 trading post in Ezekiel's possession.