Wilson’s “Carolina Cavalier” is indeed excellent.
For a nice concise history on the events leading to the War and the War itself, Ludwell Johnson’s “North Against South” is unmatched. He discusses the Sumter incident quite well, James A. Bayard, Jr., Senator from Delaware, dined with the Confederate commissioners charged with purchasing Sumter while they were in D.C. He wrote to his son that Seward was telling these men he was sick and could not meet with them. Bayard hoped Seward was telling the truth; we now know he was not.
As for books on Norther POW camps, I would suggest:
“American Bastille” by John Marshall (free) http://books.google.com/books?id=6q40AAAAIAAJ&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false
Also “To Die in Chicago” by George Levy; he discusses marshal law in Chicago as well as the horrible conditions at Camp Douglas.
“The Union Prison at Fort Delaware” by Brian Temple looks promising, though I have not had the chance to read it yet. Again, I ran across Fort Delaware quite a bit while working on my dissertation. Just a horrible place, and there were political prisoners there as well.