Das doppelte Lottchen (English: Lottie and Lisa) is exactly what I loved to read as a child. The story of the twins Liese and Lotte, who were split up soon after birth when their parents separated. Once the two very different girls realise they are twins at a summer camp, they decide to switch places.
It’s like a modern fairytale. Kästner takes elements of modern life, like parents separating, and weaves them into fairytale elements with secret twins, children’s deception and anguish and joy.
As always, Kästner has a not-so-secret agenda, but in this case it is the least obnoxious, the most human. Also the only one without a transparent self-insert. This book feels deeply like childhood to me, and I like it a lot.