This book is the kind of thing that makes wargamers start new periods! It is an examination of the Taiping Rebellion, in the China of the 1860s, from the perspective of a western soldier of fortune, Frederick Townsend Ward, who led Imperial Chinese forces against the rebels. He became a hero, general, and god among the Chinese, in a time when the "western devils" were much loathed by the Chinese as a whole. The book is 365 pages long, available in paperback, with an index, notes, and a "cast of characters." It is widely available (I bought mine at Borders, but it's on Amazon.com, too.), but don't look on the military history shelf, as it's a biography.
Anyone who read the novels The Alienist or The Angel of Darkness already knows that Caleb Carr is an excellent writer. Before he was a novelist, however, he was a military historian, and this book shows just how much fun it is when excellent history and excellent writing meet. The narrative is more fabulous than a lot of fiction, which adds to it's readability.
In terms of providing raw data for wargamers, this volume will not stand alone: it hints at much of what you'd want to know, but stops short of providing battle maps and uniform plates. (Fortunately, there is an Osprey book on the Taiping Rebellion, that will show you what the combatants looked like and which Boxer Rebellion figures you can use to equip your forces.) What The Devil Soldier is invaluable for is providing the understanding of what the combatants and leaders on both sides of this conflict were like, and how the strange (to me anyway) society of 19th Century China operated. It explains how the "Westernized" Chinese forces were equipped and trained, and how they interacted with their leaders. By providing a contrast to traditional forces, it shows how the non-Westernized units functioned as well.
Without this kind of detail, it is difficult to come up with good scenarios or realistic mechanisms for campaign games. Given the amount of "personality" that finds it's way into most Colonials games, this kind of background information can translate directly into tabletop scenarios. If you are modifying rules for the conflict, too (a necessity in this war, since the gross inequality of numbers skews most rules systems - the traditional Chinese were particularly ineffectual), you will find a wealth of detail that will help you shape your game realistically.
This is a fabulous book! The only reason not to buy it would be that you don't have the money or time to raise the needed armies to fight out yet another period... (And when did that ever stop you, eh?)