Is pricing per book or per download?

Per book — never per download. When you unlock a book, that unlock belongs to the book. You can export it, tweak it, and re-export it as many times as you want without paying again.

So a typical cover might get exported five times as you refine it, and re-exported months later when your page count changes — all on the one purchase. Same for the interior, mockups, and the rest.

See What does "per book" mean? for exactly where the line is drawn between "the same book" and "a new book."