Hey Dave,
Accessibility is on the list (but likely for Part 3) as in part 2 I also covered button states, “dribbble worthy shadows” ;-) and a couple more things. The Button chapter of the book is actually the second largest of them all (right after typography ;-) So there’s plenty to share.
As far as accessibility goes there are of course specific rules (size, contrast, safe color etc) but in general it’s best to think of accessibility as something you do for ALL users — even those without any vision trouble.
Best,