I always found this interesting, you guys may have covered it already.
http://www.ah.dcr.state.nc.us/Archives///////////news/13th_amendment.htm
I do not recollect specific discussion of that letter.
What is the text of the letter?
Later in 1861, another proposed amendment, also numbered thirteen, was signed by President Lincoln. This was the only proposed amendment that was ever signed by a president. That resolve to amend read: "ARTICLE THIRTEEN, No amendment shall be made to the Constitution which will authorize or give to Congress the power to abolish or interfere, within any State, with the domestic institutions thereof, including that of persons held to labor or service by the laws of said State." (In other words, President Lincoln had signed a resolve that would have permitted slavery, and upheld states' rights.) Only one State, Illinois, ratified this proposed amendment before the Civil War broke out in 1861.