Hans Kayser

a former teacher of mine has recently passed away. She taught German literature at university and she was one of the few inspired teachers I've met in my life. After receiving the news of her death from a friend, I have thought about her and the things she was interested in: she had an extended knowledge of alchemy (but would talk very little about it. During my last exam I asked her about a ring she was wearing and she was very surprised and promised to tell me about it the next time we would meet in the park -we lived in the same neighbourhood and would often meet there - but I never saw her again. It was a yellowish -maybe it was gold, maybe some other metal- with a square base on which a triangle containing -but the two not touching- what I took to be a cross -or the plus symbol- had been etched. Just in case someone knows about this symbol...) and was interested in Hans Kayser's theories about harmonics; I know this because I once accidentally downloaded a file meant for another course: it was an article by Rudolf Haase discussing Phytagorean harmonics. I remember my heart beating faster while reading it because of how fascinated I was with it. I promised myself to make further research on the subject but never did. The Italian translation of Hans Kayser's Lehrbuch der Harmonik seems to be out of print and impossible to find, so I will probably look for his other book on the sounds hidden in the Greek temples of Paestum. My question here is: is anyone familiar with Kayser and his work (or with Phytagorean harmonics)? Has anyone read his Lehrbuch and, if so, what does he/she think about it?