Jethro is gutted when he learns that Lincoln has been assassinated, and see finds no "beauty in the world around him or any serenity in his heart" (12.71). Lincoln was a symbol of struggle and hope for the entire country, and with his assassination as the final act of warfare, Lincoln's death is considered the worst of the worst.
Despite this, though, and despite a seriously depressing run, the book concludes on a somewhat hopeful note. After taking care of his family and the farm for the past four Aprils, Jethro gets the opportunity to leave home and receive a formal education like Shad. It's almost like the end of the war signaled freedom for all the slaves and Jeth (though we certainly wouldn't put their suffering on par), but more importantly, as Jethro runs to see his sister, "all the shadows were lifted from the April morning" (12.107). It's official: on the Creighton farm, darkness has lifted.