• Your Friend Forever, A. Lincoln: The Enduring Friendship of Abraham Lincoln and Joshua Speed by Charles B. Strozier (Columbia Univ Press, 2016).
Ever the politician, Abraham Lincoln called many individuals his friend, but his relationship with Joshua Speed was certainly one of the tightest developed during his life.
From the description: "Speed was Abraham Lincoln's closest confidant, offering him invaluable support after the death of his first love, Ann Rutledge, and during his rocky courtship of Mary Todd. Lincoln needed Speed for guidance, support, and empathy. [Charles Strozier's] Your Friend Forever, A. Lincoln is a rich analysis of a relationship that was both a model of male friendship and a specific dynamic between two brilliant but fascinatingly flawed men who played off each other's strengths and weaknesses to launch themselves in love and life."
Strozier is a historian and psychotherapist of the psychoanalytic school, and his study of the Lincoln-Speed friendship can be characterized as psychobiography. And, yes, in case you're wondering, there is an entire chapter addressing speculation on the part of other historians and writers that a sexual relationship existed between the two men. Strozier finds no evidence to suggest that theory might be true, finding instead a fundamental misunderstanding among its proponents of the nature of male bonds and cultural mores of the period (arguments that have been made before in the literature).