John Brown's Body // Treason on the Mississippi — Jack Martin

A man investigating treason is assassinated, and the case must be solved by a “brilliant and haunted Southerner” working for the Union Army (RP Dahlke, author of the Dead Red Mysteries). 

Cpt. Alphonso Clay had been summoned to meet John Brown -- not that John Brown, but a former Boston detective recruited by Abe Lincoln to put his skills to work in the war effort. Brown has now ferreted out treachery among powerful military figures . . . but he's killed before he can meet with Clay.
 
Now Clay's only hope is to decipher Brown's cryptic notes and follow his deductive instincts to solve the murder, unmask the traitors, and ensure the Union's victory at Vicksburg . . .