Österreich's family tree was reconstructed in the 20th century. His last known direct descendant was the wife of a major general in the German Army who lived in Blankenburg (Harz). She died childless in the 1950s. In the possession of the family were several letters of Österreich's along with a handwritten autobiography in his own hand. A student at the time, Adam Soltys, was writing his dissertation on Österreich in 1922. This document came to his attention and he cited it several times in his work. Despite intensive searches, the present whereabouts of the manuscript have not been ascertained. The only reliable sources of information about Österreich's life are letters and the records of the courts where he was employed. The state archive in Schleswig maintains many valuable documents from Österreich's time in Gottorf, while a majority of the archive of Brunswick-Wolfenbūttel was lost in World War II. Another important biographical source is the article on Österreich in Johann Gottfried Walther's "Musicalisches Lexicon" from 1732. Based on the detail in the article, it can be assumed that he received his information from Österreich himself. It is possible Österreich wrote his biography specially for Walther and accordingly some statements, particularly those regarding his motives for his various changes of employment must be treated with caution. Harald Kümmerling, a musicologist who has researched and catalogued the Bokemeyer collection, writing in the musical encyclopedia "Die Musik in Geschichte und Gegenwart", was critical of Österreich's work, saying "Österreich, like, Gustaf Düben was a great collector but a sterile composer." It is true, Düben's and Österreich's passion for collecting – Düben in Sweden and Österreich in Gottorf and Brunswick-Wolfenbüttel – is almost unique in the history of music. These great musical libraries are of enormous value, providing unique insight into the musical practice of the late 17th and early 18th centuries. Thanks to his collection, the works of famous composers like Dietrich Buxtehude, Nicolaus Bruhns, Johann Rosenmüller, Matthias Weckmann, Vincent Lübeck, and Johann Philipp Krieger, among others have been preserved. For Österreich, counterpoint and canon were concrete manifestations of the "order of God" ("Ordnung Gottes"), their elaboration revealing the divine, inscrutable essence of God's creation, not merely as a metaphor for His order, but as "the" concrete actualization of that order.