Charles Sumner and Lincoln could best be described as an unusual friendship formed on the strengths of their beliefs rather than on background; educated at Harvard, liberal, and a firebrand, Sumner believed in a strong central government, and in abolishing slavery. "Lincoln biographer David Donald wrote that with Sumner "the President developed bonds of personal and political friendship." According to Donald, "The relationship was difficult for both men, for there could scarcely have been too more different personalities. Handsome, Harvard-trained, and world-traveled, Sumner was the antithesis of the homely, self-educated President." (Lincoln and Friends)
Google Scholar, the Library of Congress, and the National Archives have primary resources available. A sample is listed below.