The other chapters so far are available from the companion site: http://themlbook.com/

My math is terrible, what can I read to understand this book?

There's a book "Math for Machine Learning"

Far more complete : https://www.deeplearningbook.org/ (Ian Goodfellow and Yoshua Bengio and Aaron Courville)

The book of Goodfellow et al. is about neural networks. It's not more complete than a book on machine learning.

Maybe it’s because of the 100 page limit, but it’s pretty unusual to put supervised linear methods with unsupervised clustering. Usually, those are two different chapters.

There's no clustering in this chapter.