I picked up a book at the bookstore today that I didn't know I wanted (that's one of the things I like about bookstores - just browsing around and finding stuff; hard to do that on-line!). The book is called An Illustrated Encyclopedia of Military Uniforms of the 19th Century. Here is a link to it on amazon
It gives a little bit of history for various conflicts and armies, some well-known like the US Civil War and others lesser known like the conflicts between Prussia and Denmark over the Schleswig-Holstein region. There are also sections on the Crimea war, colonial conflicts and more. The book is 9x12 inches, hardback, over 250 pages, with lots of color illustrations. It's not comprehensive by any means, but does include some "uniforms" you might not expect, such as Apaches, sutlers, and other types that you don't always encounter in uniform books. The variety of uniforms, in style, color, etc., leaves me to believe there is plenty of scope for ImagiNations in the 19th century, too.
It looks like there are enough illustrations and information for a least a basic understanding of the period/s and of what the various bits of equipment might look like (to get an idea of what colors to paint things), that I know would find useful for painting miniatures, even if they weren't exactly the units in the book.
The same publisher (and author/artists, I believe) have also done a similar book for uniforms of the American Revolution and the Napoleonic armies. I'll probably look to get the American Revolution book (and wish they had one out for the 18th century in general, similar to the 19th century one).