I'm not the parent, but I think they are referring to William Sethares' book "Tuning, Timbre, Spectrum, Scale": https://www.springer.com/gp/book/9781852337971

It's a fascinating take on how those concepts relate to each other. You can find some of the papers that were synthesized into that book on his web page: https://sethares.engr.wisc.edu/pubs.html