Watch videos (use headphones if possible) from these pages:
John Green’s Crash Course US History Video on the Civil War, Part 1 about causes, and Part 2 about effects.
Daniel Day Lewis portrays Lincoln in the movie “Lincoln” and this is a scene where he argues with other government officials about whether it is time to free the slaves by constitutional amendment.
Lincoln’s most famous speech dedicated the military cemetery of Gettysburg, read it here. Or google videos of people reading it.
Lincoln’s other famous text is the Emancipation Proclamation which declared freedom for the slaves of the Southern States. You can read the original document here. Try to understand the old fashioned English writing!