I enjoyed the blog series from NY Times: Disunion. Lot of stuff not many people know about, for example Manhattan's mayor advocating for secession, the turmoil in the south as many recent immigrants and much of the population in the hills and mountains were opposed to secession, more may be aware of Sam Houston predicting secession would result in a union victory and the end of slavery and getting removed from office as governor because of his opposition.
https://www.nytimes.com/interactive/2014...union.html