Booker writes Juliet again. He explains that he'll welcome her when she arrives, but he can't talk about his experience of the war—he shakes when he does. Booker explains that while he was pretending to be Lord Tobias, Lisa Jenkins was dating German soldiers and specifically, Sergeant Willy Gurtz. In 1944, Lisa discovered a photo of the real Lord Tobias in a magazine. She passed the photo to Willy, who passed it to the Commandant. The Germans sent John to the camp at Neuengamme.
Remember that the women who dated German soldiers often did so to feed their families; while Lisa's actions were inarguably awful, she likely told on Booker so that she herself had a better chance at surviving the war. This illustrates how desperation can turn former neighbors against each other as they try to survive.
At the camp, Booker was sent into fields to clear unexploded bombs. He kept telling himself that he was still alive, but in truth, he and the other prisoners weren't: they were just surviving. Booker was there for a year before the Germans shipped him to Belsen. There, he dug pits in which to burn and bury the dead: the Germans' attempt to hide the carnage from the approaching Allies and the USSR. Fortunately, Booker was only there for a few days before the British arrived. Those soldiers wept at what they saw and later, burned Belsen to the ground.
The Germans' attempts to hide or diminish what they'd done indicates that they knew they'd have to pay for their cruelty. It's an attempt on the part of the Germans to look more human and garner sympathy when in reality, people like Booker who saw these attempts firsthand and survived to tell the tale can share stories of their inhumane treatment and refuse to excuse the Nazis' cruelty.
Booker says that he learned that the site of Belsen is now a refugee camp. He can't quite stomach the land being used for a good purpose and feels it should be empty forever. With this, Booker concludes his account, but offers one more recollection of watching bomber planes headed for London with Elizabeth. The Germans told the islanders that London was gone, but one night, they overheard a German radio playing the London BBC station: London still stood.
Booker believes that the concentration camp sites should act as the bunkers do on Guernsey, and remind everyone who sees them of the atrocities that were committed there. This turns the camps into a text of sorts that communicates this and acts as a warning to future generations.
Dawsey writes Juliet and lists how everyone on Guernsey is preparing for her arrival. He's been trying to keep Isola alive, as Isola has been up on the roof of Elizabeth's cottage looking for loose tiles.
The mention that everyone is preparing for Juliet speaks to the strong connections that Juliet already has with her friends in Guernsey.