On the Gettysburg Discussion Group is a pretty good discussion of the book that got people hooked on the Civil War.
For me its a two part answer. The first book was Across Five Aprils, which I read at some point in elementary school. I thought the book was great. I've since found a book since it was the one that sparked my interest and while its an okay book its certainly not the best I've ever read. My interest though lay dormant until around 1992 when my best friend Dan gave me a copy of Killer Angels. His interest predated mine by a bit, he was also into reenacting which was a little odd for a high schooler. Since then my obsession with the war has grown a million fold.
From time to time I'll read Killer Angels, or at least part of it. When I needed a book while waiting for mom to come out of surgery I read it. I haven't picked it up since, but next time I need a book to kill time I'll pick it up again, starting where I left off. That's the nice part of reading a book where you know what is going to happen, I can pick it up after a month absence and be up to speed within a few paragraphs.
Then the first time I stepped on a battlefield I was hooked on that aspect of the Civil War too. I knew I had to see more battlefields. On that first trip I'm sure I drove my parents crazy with my comments about how this county highway is most likely on the same path as the road I know from 1862. I have been disappointed with some books over the years but I don't think I've ever been disappointed with a battlefield. Of sure, there are some that are too small, or do not do a good interpretation job of what they do have, but the ground is still there to walk and learn from.