Two more chapters have been added to the text: interactions and varying-intercept models.
The interactions chapter describes what linear interaction terms are and demonstrates a model with a continuous-categorical interaction. An example of a continuous-continuous interaction is missing and remains TODO -- the limiting factor is finding an actual example!
The varying-intercept model has two parts: conceptual + notation and "how-to fit in brms and interpret output." I had a very hard time writing this chapter and it needs work. One of the limitations is that the example I used is not excellent: very few categories and not a lot of shrinkage. Will revise.