Textbooks and graded readers are the obvious ones to set at specific levels, but I just remembered this thread: Books translated into multiple languages, which might be useful.
I don’t know how many books you’re looking for, but I think these ones would be good:
Der Kleine Prinz (Mit den farbigen Zeichnungen des Verfassers) | L22??
Das magische Baumhaus (series) | L22??
There are some I think that will be useful to calibrate new languages in the future, although they haven’t been graded enough in Japanese yet, specifically:
- Lord of the Rings
- The Chronicles of Narnia
