There is a pretty good little write-up on the subject of Civil War Spies on my website. Here's the link to the page.
If you're looking for some interesting accounts, see if you can find info. on Franklin Thompson, 2nd Michigan, Co.F. Frank's real name was Sarah Emma Edmonds(Seelye, after she married). Also look up Harry T. Buford, aka Loreta Janeta Velasques, who was a spy for the Confederacy. Both of these women masqueraded as men,fought in battles, and worked as spies. I don't know if there's any info on the web about them, but 'Patriots in Disguise' by Richard Hall tells their stories, and some others.