Hey Guys.
There is another book in the Jim Knopf series: Jim Knopf und die Wilde 13.
Further more I can recommend to watch out for "Die Augsburger Puppenkiste", which is a puppet show that (among others) covered the Jim-Knopf-Adventures by Michael Ende. "Die unendliche Geschichte" which was mentioned before is definetly a great book and dont miss to read "Momo" and "Der satanarchäolügenialkohöllische Wunschpunsch", both written by Michael Ende as well. (no worries: "satanarchäolügenialkohöllisch" is actually an experimental neologism, but it illustrates how it´s possible to mash up words in German language.)
Another book series I can really recommend is "Urmel" by Max Kruse. It´s a bit special since the characters all have a different speech impedement, but in my experience it´s a great occassion to think about language. By the way, "Urmel" was also covered by "Die Augsburger Puppenkiste".
Best wishes,
Peter