Lincoln lived long enough to experience Lee's surrender, which was the pivotal event in the south's defeat. He also worked hard to shepherd the 13th amendment through Congress. By the time it passed the House, he & they knew its ratification was a foregone conclusion.
Does Bennett's book also point out that Garrison, Greeley, Beecher & other abolitionists were also pro-secession? Can you imagine what would happen to black people in the south had it been permitted to secede & form a slave superstate in the south & Caribbean, as was planned?
I think the evolution of Lincoln's POV re abolition & black suffrage is strikingly similar to that of another self-taught leader & orator, one who was also murdered, exactly one-hundred years later. Malcolm X.