“Recreational Mathemusician” Vi Hart is the God of this fun maths and music stuff:
https://youtube.com/@vihart
On the more hardcore side without much of the fun:
The Musimathics books by Dr. Gareth Loy which are a bit much for me but I have them on my shelf and will try again one day:
http://www.musimathics.com/
And The Geometry of Music by Dmitri Tymoczko which is pretty readable:
https://dmitri.mycpanel.princeton.edu/geometry-of-music.html
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