And the king left ten women, which were concubines (2Samuel 20)

While 2Samuel chapter 20 is more action-packed than the previous chapter, there's not a lot here to cover. The fallout of the argument from the end of the last chapter is that a man named Sheba blows a horn and yells to the Israelites that David is clearly king of just the tribe of Judah, and the other tribes should abandon David, so they all leave. As almost a side note, David takes the ten concubines that Absalom had sex with and puts them away somewhere separate where he never has sex with them again.

David tells Amasa to assemble the troops and go after Sheba for being a troublemaker. They go after him, but on the way, Joab kills Amasa. (Joab really has this tendency to kill people who have offended him and/or David against David's will.) When they come to the city where Sheba is, they lay siege to it, and try to break down the wall. A woman comes to the wall and asks Joab what the meaning of the attack is, and Joab says that they really just want Sheba. So the people of the city chop off Sheba's head and throw it down to Joab. Joab blows a horn, and the army goes back to Jerusalem.

They brought him to Jerusalem (2Samuel 19)

The SAB marks 2Samuel chapter 19 as "boring", and yeah, it probably is; but what do you want, more mass killing and rape?

David is still mourning Absalom, and Joab goes to find him and says essentially that it's looking bad that he's mourning someone who wanted him and his family and friends dead, and if he wants to stay king, he needs to buck up and go talk with the people. So David goes and sits by the gate of the city, a traditional place for leaders to be, and the people notice and come to him. David sends word to the tribe of Judah (his own tribe) and asks why they haven't sent for him to come back, and tells Amasa that he will be the head of the army in Joab's place. They send for David, and he goes back to cross the Jordan.

A bunch of people show up at the crossing. Shimei shows up with a thousand Benjamiites and apologizes for the way he treated David on the way out. Mephibosheth shows up and says that he has been slandered by his servant. David is not in a mood for revenge at this moment, so he forgives them. An 80-year-old man named Barzillai, who helped David when he was in exile, shows up to see David back to Jerusalem, at least part of the way. David returns to Jerusalem, and there's a bit of an argument between the men of Judah and the men of the rest of Israel over who is more loyal, which apparently leads to problems in the next chapter.

King David mourned many days for his son (2Samuel 18)

2Samuel chapter 18 opens with David finally organizing his retaliation against Absalom. He apparently still has thousands of men on his side, and he divides them into three armies. The men tell him to not fight because his life is more important than theirs, and so he stays behind. 20,000 men are killed, probably mostly by David's soldiers. I think that verse eight is really saying that the slaughter was worse just because they were fighting in the forest, not that the forest literally killed people.

Now Absalom comes riding on a mule, and he gets his head caught in some tree branches. This is supposed to be ironic, because one of the things people admired Absalom for was his huge head of hair, and it proves to be his undoing. So somehow, he's left there by himself just hanging by his hair, and a man reports this to Joab, who gets angry with the man for not taking advantage of the situationa5 and killing Absalom. The man replies that David told everyone that no matter what happens, not to hurt Absalom. Joab clearly doesn't care, and stabs Absalom in the heart with three arrows; ten men with him also attack Absalom. Joab blows a trumpet and the fight ends. They throw Absalom's body in a pit and cover him with stones. I addressed whether Absalom had children in 2Samuel chapter 14.

A man wants to run to David and tell him the news, but Joab stops him, because he knows David won't be happy (and perhaps he wants to make sure the king doesn't know who struck the killing blow), but then he tells a different man to run and tell David. In the end, both men go, David finds out, and he appears to be heartbroken.

And Ahithophel said unto Absalom (2Samuel 17)

So, there's not much in 2Samuel chapter 17 so I'll make it short. Ahithophel suggests a battle plan to take down David. Absalom wants to hear what Hushai thinks, and Hushai suggests a different plan. Everyone likes Hushai's plan better, supposedly because it's God's will for Ahithophel to be a failure. Hushai sends word to David to plan accordingly, and David successfully hides. Ahithophel commits suicide. The SAB marks this suicide as injustice, but I don't know what's unjust about suicide, as one does it to oneself. In my opinion, this is more absurd than unjust; I mean, Ahithophel gets rejected a total of one time, and he ends his life over it? What the heck? Who was Amasa'a father? I notice that in this chapter, Amasa's father is said to be Ithra, while in other places, it's Jether, but Ithra and Jether are just alternate spellings of the same Hebrew name. I don't think there's an easy answer, it's just an error in one place or the other.

Shimei the son of Gera, a Benjamite (2Samuel 16)

There's an interesting vignette at the beginning of 2Samuel 16 with Ziba, the servant of Mephibosheth. He shows up with a bunch of supplies for David, and David asks where Mephibosheth is. Ziba explains that apparently Mephibosheth is taking Absalom's insurrection as an opportunity to get political power (for who knows what reason), and David tells Ziba he can have all of Mephibosheth's property.

Then we see another interesting vignette with Shimei, some relative of Saul, who comes out to yell curses at David and throw rocks. Abishai asks to go over and kill Shimei, but David says to leave him alone, because maybe he deserves what Shimei is doing. This is interesting, because Saul was told to kill Agag, and failed to do so in a timely manner, and Haman, an Agagite, tried to kill the Jews in the book of Esther, but he was thwarted by Mordecai, a descendant of Shimei (see Esther 2:5). The SAB does point out that David asked Solomon to kill Shimei when he was on his deathbed, but perhaps by that time, he felt it was clear he had God's favor, and clearly Shimei was not doing the will of God, but just being an asshole.

So the chapter ends with Absalom arriving in Jerusalem, greeted by Hushai, David's spy. Hushai assures Absalom that he serves whoever the people choose as king. Ahithophel suggests to Absalom that he have sex with the concubines that David left behind, which, as I've pointed out before, was a common thing for new kings to do when they had conquered the former king. Indeed, as the SAB points out, this was foretold in chapter 12, and was said to be the consequences of what David did with Bathsheba. (See my interpretation of this "punishment" there.)