Stephen Berry, who did a book on the Todds a few years back, and Angela Esco Elder have just published
Practical Strangers: The Courtship Correspondence of Nathaniel Dawson and Elodie Todd, Sister of Mary Todd Lincoln, a compilation of the couple's correspondence before their marriage in 1862. I scanned many of these letters for my own research before leaving NC, and they make for fascinating reading. The authors also plan a website where one can access the rest of the couple's correspondence and related letters.
https://www.amazon.com/Practical-Strange...+Strangers