Jonathan W. White, Emancipation, the Union Army, and the Reelection of Abraham Lincoln, (Louisiana State University Press, 2014). This is a must read. White challenges long-standing views about the support within the ranks for Lincoln and the Republican Party in 1864. His analysis of the extent to which the Lincoln administration and Union high command suppressed dissent in the ranks is also very interesting.
I also want to highlight a new book co-authored by Michael Musick called “I Am Busy Drawing Pictures”: The Civil War Art and Letters of Private John Jacob Omenhauser, CSA, which you can pick up from the Friends of the Maryland State Archives. Omenhauser spent time at Point Lookout Prison. While his letters are insightful, the real prize are the incredibly rich images that detail life in prison – some of the most interesting focus on race relations and the humiliation of being guarded by black Union soldiers.