In short I think it’s crucial to get an accurate (not “lost cause of the valiant confederacy”) appreciation for how bloody it was and the real stakes (slavery, not “states rights”). How progress was made politically. How there were a few years of positive change before the US backslid into racial patterns of old due to moral exhaustion fighting the south.
The period after Lincoln's death until Grant's election is nearly unbelievable. Johnson was an avowed racist and openly apologistic to southern gentry. The south and a sizable percent of the population was under armed guard and were essentially in military dictatorship under Grant. Grant was the obvious next pick for president and, wisely, was quiet about being the 'real' power in the US, physically right next to Johnson.
Then, as Johnson can't help himself but to be a bullheaded moron, he gets impeached by the radical left wing of the house: the 'newish' Republican party. His trial is wild, by the way. He gets impeached, and is then sent to the senate. Where the southern states, still under the war department, can't vote or sit; it's all Union states. Bribing was rampant in the senate, but not publicly known. Johnson misses conviction by one vote. The left-wing Republican senators that vote to acquit never serve in public office again.
Again, Grant is a dense read, but Chernow did a fantastic job on it. Big recommend