I ran across the letter years ago when doing some research on the New York convention. It was a private letter—I don’t remember who the recipient was. For sure the question of whether a state had a right to secede was unresolved before the Civil War, and only resolved by force by the Union army.
Prior to Lincoln, just about everybody agreed states had the right to secede. Lincoln himself praised secession as “a principle to liberate the world” in 1848 in a speech as a US Representative.