After his doctorate Kapp published several books on Hector Berlioz, Franz Liszt, Carl Maria von Weber and Giacomo Meyerbeer (1920).
[1] In 1937 he published a Geschichte der Berliner Oper, to which Hermann Göring wrote a preface.
In Nazi Germany the opera Nabucco was undesirable because of the theme from the history of Israel.
In the choir Va, pensiero he replaced the Jordan with the Nile and Zion with Memphis.
[2] This NS version Flieg, Gedanke, getragen von Sehnsucht with the text line Teure Heimat, wann seh ich dich wieder is still widespread today in sound recordings and on the Internet.