This stamp belongs to the library of the teachers union in Germany, which exists up until today. (With the interruption of the Nazi era.) This stamp is on a journal on youth education from 1911.
The stamp reflects the modern style of layout of the time, as the union itself is still know to be one of the more progressive unions. This holds also true for the 1910s and 1920s.
Interestingly the modern style of layout is not reduced to the library stamps, but also used at the layout of the journal itself.
Another close look.