So there's a companion PDF to How to Design Programs (HtDP) called How to Design Worlds (HtDW). And I'm wondering how much of HtDP do I have to read/know to be able to start reading HtDW? Are the chapters from the two books correlated in any way?
Try reading Chapters 1-4 of HtDP 2e
HtDW is meant to be a companion to HTDP, in the sense that the early stuff in it doesn't require much at all (The prologue, Chapter 1, of HtDP 2e), since it's just limited to basic arithmetic and images. But other material in it depends on design programs that work with arbitrarily-long list data, and that's Chapter 4.