Hi, Larry, sure-one is Christopher Duffy's "The Army of Frederick the Great", which, though it is a study of the Prussian army, provides a wealth of information about the rest of Europe's military as well.
On Revolutionary War uniforms, John Mollo's "Uniforms of the Revolutionary War" is an excellent reference, as is Mollo's "Uniforms of the Seven Years War."
Several Osprey volumes are very useful for the 18th century, in this case, "The British Army in North America", provides some details about the German units as well. Also the recent, "Frederick the Great's Allies" provides a nice look at the Hessian regiments serving in central Germany in the Seven Years War, and describes the reforms that the Landgraf implemented towards the end of that war, which maintained till the beginning of the French Revolutionary wars.
In German, a book that is particularly valuable is Karl Trenkle's "Nix Wie Weg-die Hessen komme!" ("Run Away! The Hessians are Coming!"). Trenkle (say, "trenk-leh") describes the history of the army of Hessen-Cassel, then the histories of each regiment in each arm of service, including the names of the
Chefs or colonels-proprietor, from the Seven Years War until the old army was disbanded in the defeat in the Napoleonic Wars, but also notes about the later Imperial regiments that inherited their traditions. Illustrations are also provided, as what appear to be water-color sketches.
Pengel & Hurt's notebooks on the various armies in the Seven Years War are also valuable, though I find them hard to read, and they apparently cite most of their material from Friedrich Schirmer's "The Armies of the Combattant States in the Seven Years War", printed through the German Klio society in the early 80's, and also containing some assumptions or even errors, which Pengel & Hurt propagate, but which have since been clarified or corrected by newer info.
Two websites are also very useful. One is the French site Nec Pluribus Impar,
http://vial.jean.free.fr/new_npi/, (which was the Sun King's motto, if I am not mistaken). It used to be updated/published monthly, but its editor cut back to quarterly updates a couple of years ago. But it has a great collection of illustrations and information about the armies in the wars in the 1730s through the Seven Years War.
The other website is the relatively newer Project SYW, at the Kronoskaf website:
http://www.kronoskaf.com/syw/index.php?title=Main_Page. There is a lot of good information there, too, organized by state, with many contributions by members in Germany and France, with access to obscure original sources, and there are more updates all the time.
Those are some of my favorites, though I do have more hard copy in my library and in my Favorites list, but time and space prevent me from listing more.
Hope those help, prosit!
Brad