Those Osprey books have great plates. They also have some great photos for uniforms and flags.
However, you should note that a vast majority of the uniforms they do plates for were peace time or dress uniforms and not actually ever worn in battle (except for maybe the first battles, Manassass, Wilson's Creek). The 11th NY Fire Zouaves are a good example, in that their well known dress uniforms were all ditched prior to 1st Manassass because they were too hot to march in so they fought in undershirts and regulation trousers.