A package of Dover reprints of classic reference books:
1. Revised Regulations for the Army of the United States, 1861 issued by the War Department, US (unabridged 2013 reissue of 1861 ed. by J.G.L. Brown).
This is the must-have handbook that Civil War volunteer officers pored over to learn their new duties. I can't say that I've ever looked through it before. "...the guide includes many official forms used in the 1860s as well as an appendix containing articles of war, extracts from Acts of Congress, and an Army pay table. Contents include regulations related to military discipline and rank and command, care of fortifications, treatment of deceased soldiers and deserters, leaves of absence and furloughs, arrests and confinements, inspections of the troops, musters, and forms of parade. Additional subjects include troops in campaign, courts-martial, recruiting, medical and ordnance departments, flags and standards, uniform and horse equipment, and many other practical considerations."
2. The Art of War by Antoine-Henri, Baron de Jomini, trans. by Capt. G.H. Mendell and Lt. W.P. Craighill (unabridged 2007 reissue of 1862 J.B. Lippincott ed. - 3 orig. pull out maps reproduced in reduced size format).
I own a copy of what's said to be the best and fullest English translation of Clausewitz's On War (the Howard and Paret Princeton edition) but don't know if the Mendell and Craighill edition of Jomini is similarly regarded. I'm a little embarrassed to admit that I've never read either work in their entirety. Jomini looks very accessible.
3. Arms and Equipment of the Civil War by Jack Coggins (2004 repub. of 1962 Doubleday ed.).
A Centennial classic, Coggins's book apparently inspired many budding Civil War enthusiasts and historians. It is certainly a common occupant of Civil War sections inside used bookstores all across the country. A heavily illustrated primer, the book discusses the basics of Civil War infantry, cavalry, and artillery as well as their associated engineer and signal corps detachments. Railroads, the medical and quartermaster departments, and the navies are also introduced.
4. Civil War Collector's Encyclopedia: Arms, Uniforms and Equipment of the Union and Confederacy by Francis A. Lord (2004 reprint of 1963 Stackpole ed.).
Another bookstore mainstay, this is the granddaddy of Civil War collectible guides. Over 800 items are organized topically and described in minute detail.
5. Uniforms of the Civil War by Francis A. Lord (2007 reprint of 1970 Yoseloff ed.).
This one is more obscure to me. Of course, much more information has emerged in the 45 years since original publication, but it still looks like a serviceable pictorial introduction to the subject. I wonder if the painted figures guys still use it as a guide.