A newly discovered letter written by former first lady Mary Todd Lincoln shows a side of her that is rarely seen, an author and historical document expert told FOX Business.

The letter is addressed to George Harrington; his descendants reportedly found it in their family archives. At the time, Harrington was assistant secretary of the Treasury. 

In the letter, Mrs. Lincoln was trying to get a Treasury Department job for a woman, which Nathan Raab told FOX Business was very unusual for a woman during that time period. 

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"It's remarkable to see her working within the context of the government during the Civil War, working to get women involved in the war effort and government while we were fighting the Confederacy," Raab said in a phone interview about Mrs. Lincoln. 

Raab is president of The Raab Collection, a firm that sells historical documents, and is the author of "The Hunt for History." He's located in Philadelphia. 

The letter, believed to be from 1864, shows former first lady Mary Todd Lincoln engaged in an effort to find a job in the Treasury Dept. for an unnamed woman. (Courtesy of The Raab Collection  / Fox News)

The Raab Collection is selling Lincoln's letter for $15,000. 

The letter is believed to be from 1864.

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In the letter, Mary Todd Lincoln asks Harrington if she can suggest a new person to hire for a role at the Treasury, as the original person "has not acted worthily."

Mary Todd Lincoln, right, was the wife of President Abraham Lincoln. A newly discovered letter from her shows a different side of her than what's normally seen, an author told FOX Business.  (Stock Montage/Getty Images / Getty Images)

"I am sure it will be an unimportant matter to you, and your compliance will gratify me," wrote Mrs. Lincoln. 

The person referred to in the letter is unknown; it's also unknown if the person was ever hired by the Treasury, Raab told FOX Business. 

"Something about this letter feels warm and touching."

"I think [it's] more from the perspective of, well, here she is behind the scenes working on behalf of women, plural," he said. 

"It shows that she had a philosophy of helping employ women in the war effort and in her husband's administration." 

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Further, the letter goes against the idea that Mrs. Lincoln was aloof and unfeeling, said Raab.

"Something about this letter feels warm and touching," he said. "It brings out a side of her that I'm not sure I had realized," he said.

The letter provides new insight into Mrs. Lincoln's personality, said an expert in historical documents.  (Universal History Archive/Universal Images Group via Getty Images / Getty Images)

Normally, "we see Mary Todd Lincoln through the lens of Abraham," said Raab.

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This letter, he said, "gives her own perspective." 

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"And we can see what she was doing, not just her commenting on what the president was doing," said Raab. 

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