Hogwarts-Schule für Hexerei und Zauberei
If there are any other Harry Potter fans out there, such as myself, who are looking for a first German book to read I would strongly suggest Harry Potter und der Stein der Weisen, which is the first book in the series. It is a fun and relatively easy read set to a familiar story, and there is also tons of new vocabulary that isn't in the Duolingo course such as Zauberstab (Magic Wand) and Verwandlungen (Transfiguration)! And, if you aren't looking for a German edition of the book,the Harry Potter series has been translated into over 67 languages, including all of the languages found on Duolingo (With the exemption of Klingon), so you can even read it in Irish or French if you wanted to!
Viel Glück und hab Spaß!