Would questions concerning illustrators/graphic designers for books be on or off topic?
We can approach this in a way that's analogous to bicycle touring questions on the bicycling SE site, which can potentially involve questions about riding and camping. If a question involved camping, it would be off-topic, but anything about the riding part of the equation would be okay.
Similarly, we should include questions about illustration and layout in our scope, only when they relate to writing. So, "How do I illustrate/choose a typeface for my book" would be out of scope, but questions like "does choice of typeface change the users' experience of the text" or "can I cut down on description when I know there will be illustrations" make perfect sense for this site.