If comics and illustrated stories count I’d like to add some (from the past 40ish years), most of which also come as movies, TV series or audio plays and as toy merchandise:
- Struwwelpeter by Heinrich Hoffmann – a cruel classic.
- Oh, wie schön ist Panama and other stories by Janosch – You can hardly consider yourself German if you don’t know what a Tigerente is (except if you’re old).
- Freunde, Mullewapp, Tabaluga, Sauerkraut and other books by Helme Heine
- Werner by Röttger „Brösel“ Feldmann – It’s a challenge due to written (Northern) dialects, and not that suitable for younger kids.
- Käpt’n Blaubär, Das Kleine Arschloch and other stories (for older kids and grown-ups) by Walter Moers
- Fix und Foxi by Rolf Kauka
- Ritter Rost by Jörg Hilbert / Felix Janas; includes music.
- Urmel stories by Max Kruse. The puppet version by Augsburger Puppenkiste actually may be better known than the books.
- Pumuckl by Ellis Kaut. The original radio plays and television series are more popular, though.
- Benjamin Blümchen and Bibi Blocksberg by Elfie Donnelly (who’s actually British). These are mostly known from audio plays.
- Ein Fall für TKKG by Rolf Kalmuczak. Also maybe better known from the audio plays.
- Prinzessin Lillifee by Monika Finsterbusch – pink as hell, like German Hello Kitty
- Käpt’n Sharky by Jutta Langreuter / Silvio Neuendorf – male counterpart to Lillie
- Die Olchis by Erhard Dietl
- Biene Maja originally books by Walter Bonsels, but most only know the animated series – as a German you know the lyrics to the theme song.
- Wickie by Runer Jonsson (who’s Swedish) – like Maja best know from a 1970s anime adaptation.
- Felix by Annette Langen
- Bobo Siebenschläfer by Markus Osterwalder
Asterix, Lucky Luke and others may not be as well known in English-speaking countries, but are popular throughout Europe and they are not of German origin, but French/Belgian.
There’s also a variety of Wimmelbücher (e.g. by Ali Mitgutsch) without text that are not as popular in other countries as far as I know, with the notable exception of Where’s Wally? (or Waldo).
Likewise cheap 10 cm × 10 cm Pixi-Bücher are well known among German children and parents. They feature some original characters like Pixi, Conni and Petzi, but also other better known stories that have been shortened or edited to fit the format.
Recent book series, not comic