The author Julius Ruska died in year 1949. The copyright has expired in jurisdictions where the copyright is the lifetime of the author plus 70 years after the death of the author.
The last one-third of the book is in medieval Arabic. The middle one-third of the book is a translation of the medieval Arabic into modern German. The first one-third is intro and commentary in German. The medieval century in which the medieval Arabic was written is uncertain.