Kinda surprised I haven’t seen it here: Die Känguru-Chroniken by Marc-Uwe Kling. It’s about a Kleinkünstler (argh!) living together with a communist kangaroo. They get up to all kind of nonsense whilst the book satirical looks at german culture and politics. It’s quite popular. The sentences are often pretty short and straight to the point as are the chapters, so you can pace it quite well. If you like the style of the books and would like to practice your listening comprehension, I can also recommend the audio book, read by the author himself. His voice-acting takes it to a whole new level.

“Momo” is a book you can read as a kid (hence easy to read) but so deep, you will only get it as an adult

Ottfried Preußler has allready been mentioned.

I’d like to add Erich Kästner (probably more children’s books, but imho still very enjoyable).

Michael Ende, the Neverending Story and Momo are classics for a reason. checkout “Der satanarchäolügenialkohöllische Wunschpunsch” it is a childrens book, but very funny and i actually read it first when i was like 13 and still loved it.

Paul Maar, “Eine Woche voller Samstage” and the subsequent Sams books are great, but again more children’s books than YA.

Walter Moers, the Zamonien Books where a big hit in my nerdy friend circle during school. the closest thing i can think of would be Discworld.

Wolfgang Hohlbein, he writes a lot, most of it is not that good, but some of his books are great and the books he has written with his wife Heike tend to be better. I enjoyed as a teen( and these are definitely YA-fiction stuff):

• Der Greif
• Das Buch
• Anders, a 4 Book series
• Märchenmond, 3 Book series

I am not looking for children books

What about books for youths?
Krabat by Otfried Preußler is one of the best youth books, but also just a very good fantasy novel in general.