@newt @scathach
I'd recommend Musimatics by Gareth Loy: https://www.amazon.com/Musimathics-Mathematical-Foundations-Music-1/dp/0262122820
I own both volumes, but I've never touched the second one — I don't think it gets worse after the first, it starts with the basics, but advances quickly and covers a very broad range of topics associated with music.