I suggest a copy of Hardtack and Coffee by John D. Billings, Company Aytch by Sam Watkins, and Soldiering by Rice C. Bull. Not sure if your impression is going to be north or south, so I offered two titles from the northern persuasion and one from the south.
It is wise to not purchase anything until you know the requirements for your unit. However, building knowledge of the life of a soldier is priceless, and these title will do a great job getting you "started."
I also recommend strongly that you read as many journals/diaries and letters that you can find online. A simple Google search can find you mountains of information.
I hope this is helpful to you.
David M. Jahnke
\"We talked the matter over and could have settled the war in thirty minutes had it been left to us.\"