Mary Todd Lincoln was an interesting lady. The library has several books about her, including a biography.
Mary Todd Lincoln wrote many notes of encouragement and advice to her husband. You can locate some of her original documents and samples of her writings at the Library of Congress.
Some examples of items you can find at the Library of Congress online are:
The Role of the First Lady and the Media: A Preliminary Case Study of New York Times Coverage of Mary Todd Lincoln, 1861-1865. In The Civil War and the press [electronic resource] / David B. Sachsman, S. Kittrell Rushing, Debra Reddin van Tuyll, editors ; with Ryan P. Burkholder.
Note: not all of these articles are available full text online. Some may need to be Inter Library Loaned. Not all may be appropriate for your topic.