I happened to pick up Osprey's "Frederick the Great's Allies" on Saturday, from the Men at Arms series, I've wanted to have a look at it, since it was released in September. It covers the armies of Hannover, Brunswick, and Hessen-Cassel. The text is organized by arm of service, and then under each rubric: Infantry, cavalry, light forces (including light infantry and light cavalry), and technical arms, each state's forces are described.
It's not a bad reference, though most of the information is already available in other sources. For example, many of the illustrations are by Knötel; I have them in my collection of German cigarette cards. Also, there is no information in the text about the flags of any of the states, though there is some info in some of the illustrations' captions.
The only other comparable reference, that collects this same info into one volume, is the one put out by Partizan Press, and this Osprey volume is much better. The Partizan book includes some info that is now known to be wrong, such as descriptions of the Brunswick colors. There are a couple of minor errors in the Osprey book, though, too. One is a style choice--the author refers to the Imperial armies as "Imperialist" armies. As far as I know, "imperialist" has a very specific meaning in English, and the better word should be "Imperial". "Kaiserlich" or "Reichs-" are best translated as "imperial". Also, they get the name of the Chef of the Death's Head Hussars wrong, it's "Ruesch", not "Reusch". A minor error, but in my other Osprey books, they got it right.
Still, I'm glad to have it, and it's given me ideas for some of the castings currently on my bench and in the queue. I recommend it, for any fans of the Seven Years' War. I'd also pass on the Partizan book, if I were you.
Prost!
Brad