In addition to reading Volume One of Bruce Catton's ACW opus (which gave me a good feel for the causes of the war), as we played War Between The States I read this book: 1400 Days by Chris Bishop, Ian Drury and Tony Gibbons.
The illustrations are excellent and the day by day account gives a good pace and also provides some wider context (as does the introduction and the end piece).
I know I bought my copy cheap, years ago. I now know why. It is missing pages 161 to 176. This means it skips from 25 November 1863 to 24 March 1864. Boo!
Next up is to read Terrible Swift Sword, Volume 2 of Bruce Catton's trilogy.