Secession
Free to read here, from Project Gutenberg.
The American Republic: Its Constitution, Tendencies, and Destiny
Webster's Seventh of March Speech and the Secession Movement, 1850
Nullification, Secession, Webster's Argument, and the Kentucky and Virginia Resolutions
The Crisis of Eighteen Hundred and Sixty-One in the Government of the United States.
Baltimore and the Nineteenth of April, 1861: A Study of the War