Would questions concerning illustrators/graphic designers for books be on or off topic?
Yes if it is about writers working with these professionals.
Yes if it is questions from these professionals that relate to writing or publishing.
No if it is questions from these professionals that relates to their work. There is an SE site in the works for them.
I'd think it would be on-topic, especially if it is involved in the process of contacting and contracting such professionals. Recommendations on firms or individuals to use, however...
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.