I have a large library of reference books about military uniforms and can suggest any number of books. A lot depends on if you are asking about uniforms, insignia or weapons. For Civil War uniforms you might want: Army Blue The Uniforms of Uncle Sam's Regulars 1848-1873 by John P. Langellier (you might find a used copy for around $45). This covers the U.S. Army very well but skips militia uniforms. I find American Military Equipage 1851 -1872 by Frederick P. Todd very useful as well, but it is a bit dated and new information has been discovered. You will want the 2nd printing which was done in two volumes (out of print and rather expensive at perhaps $80 per volume used) but if you are rich go for the four volume 1st printing with the extra color plates (perhaps $250 for the set when and if you can find a set). Todd covers militia uniforms fairly well. For headgear I would highly suggest U.S. Army Headgear 1812-1872 by John P. Langellier and C. Paul Loane (a bit pricey at $70 but will save you purchasing several books on the subject). I find United States Army Headgear 1855-1902 by Edger M. Howell useful, especially for quick references to Army regulations about headgear. (kind of rare but a used paperback copy should go for $15 or so). If you are interested in insignia I would suggest you do a book search for books by William Emerson (unless you are very serious I would skip his book on chevrons which goes for $350-$500 used, when and if you can find it, I have told Mr. Emerson that I was outraged by the cost of his used books). There is a new book about GAR insignia and uniforms, but I have not purchased a copy and the name escapes me.
If you are interested in Confederate uniforms, Marine Corps uniforms or Naval uniforms send me a PM and I can make some suggestions.