Henry T. Shanks, The Secession Movement in Virginia 1847-1861. I have the AMS edition published in 1971, which is a reprint of a 1934 edition, which as I recall was a flushing out of a Ph.D. thesis. Doesn’t seem to be available on Google Books from a quick search.

The book (as you may have guessed from the title) focuses mainly on Virginia, but there are some comparisons with other southern states, principally South Carolina.

There is also a book on “evangelists” sent by seceding states to other slave-holding states to convince them to join in the secession movement. I cannot think of the name of it, but I may have a copy of it.

This might have to be approached state by state. For instance, see documents compiled in Secession Debated: Georgia’s Showdown in 1860 edited by William W. Freehling and Craig M. Simpson. There are collections of house/senate journals for several states at ^[1] (see “Secession” under Subjects)

This might be promising, but I’m not familiar with it:
Ralph A. Wooster, The Secession Conventions of the South (Princeton, New Jersey: Princeton University Press, 1962)

Edward is thinking of Charles Dew’s Apostles of Disunion: Southern Secession Commissioners and the Causes of the Civil War. I haven’t read it, so I can’t say how useful it might be to you.

You might take a look at some of the books mentioned at the very end of the post linked below. Unfortunately all the links in the post seem to have become corrupted, but you should be able to find the books with a quick search of Amazon. Charles Dew’s fine book is included.

^[2]