The German Language

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

"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

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.

So... I guess the first question is if the books should be written in German or if a good translation is enough. I'll assume the latter and just won't look whether a book was written in German or just translated.

Books aimed at children are a whole industry, more or less. Kids that learn how to read are their own target group, so you'll find any genre you want. However, as the children are supposed to easily identify with their protagonists, they are often about school children doing stuff. More often than not about kids that read a lot and are quiet outsiders. If you can't bear with that trope it'll be a bit harder, but still manageable.

For criminal stories, there are some franchises that have been going on since more or less forever... "???" ("die drei Fragezeichen" and their "kids" spinoff that is targeted at younger audiences) or "tkkg" come to mind. A very German classic is "Emil und die Detektive", which plays in Berlin in the 1920s and holds up very well.

If you're more into fantasy, maybe Michael Ende's more famous works are more to your liking. "Die unendliche Geschichte", "Momo" or "Jim Knopf und Lukas der Lokomotivführer" are classics as well and just excellent. Cornelia Funke also wrote neat books, "Tintenblut" or "Drachenreiter" work really well. Recently, "die Schule der magischen Tiere" was pretty successful and my daughter loved the series.

Astrid Lindgren has good translations. All her stories are terrific. Pick anything. Some "Michael aus Lönneberga" book maybe.

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.

If you're not opposed to German translations of English books Terry Pratchett's YA series centered on Tiffany Aching is pretty engaging. The first in the series I believe is "Kleine freie Männer", but a German speaker can correct me on the title and even recommendation