David Miller's The Illustrated Directory of Uniforms, Weapons, and Equipment of the Civil War is regarded as the gold standard work on Civil War militaria.

Robin Smith and Ron Field's Uniforms of the Civil War: An Illustrated Guide for Historians, Collectors, and Reenactors is more of an entry-level text, but is very accessible.

You may also find it worth buying one of Don Troiani's art books. He does meticulous research into his Civil War scenes and does a great job capturing the essence of Civil War soldiers in camp and combat.