The Loyalty Oath during the Civil War

[Wild Bill]

I claim no expertise on the Oath of Future Loyalty or the Ironclad Oath (never bore arms for or assisted the Confederacy in any manner), but here are two books by expert Harold M. Hyman:
Era of the Oath: Northern Loyalty Tests during the Civil War and Reconstruction (University of Pennsylvania Press, 1954, reprinted, Hippocrene Books, 1978).
To Try Men's Souls: Loyalty Tests in American History, (University of California Press, 1959, reprinted, Greenwood Press, 1981).