I just discovered this article from way back in December 2010 in which the author lists his Top 12 Civil War books of all time. As he readily admits, it is highly subjective and is assembled with some ground rules such as no biographies, no fiction, nothing published prior to 1950, and no multi-volume sets, which obviously leaves out a lot of classic titles.
Given those rules, how many of these would be on your list?
While all of these may be fine books, none of them are difficult to aquire as first editions, with the Catton and McPherson probably being the most expensive. One interesting note is that the 1954 Pulitzer Prize-winning Stillness at Appomattox was reissued in 1982 by the Book-of-the-Month Club as an oversized hardcover in slipcase (see pic) that can be a handsome alternative to the first edition. Pictured copy offered here.