This stamp is historic. The library (“Vadiana”) today is not the Stadtbibliothek (city library) of St. Gallen anymore, but the district library of the canton of St. Gallen (named after the city of St. Gallen, but much bigger). [We covered some stamps from the today-Vadiana on this blog before] The public library today is the “Frehandbibliothek”, separated from in “Vadiana” in the 1970s. In a strange turn of events, those both libraries are on a way to merge again into a new library which will contain the Vadiana and parts of the “Freihandbibliothek”. This stamp, used in the 1920s, states the Vadiana as the city library.
The stamp has a quite unusal form. It reminds of coats of arms, also on garlands of corn. Most likely it is formed after the embroidery, which was a major industry in Switzerland and especially in St. Gallen around the time the “Vadiana” was founded.