us code 106 gives the copyright holder exclusive rights of derivative works
so yes even at home, making art for a book you dont have permission for, and isnt seen by anyone but you, is still copyright infringement
so yes even at home, making art for a book you dont have permission for, and isnt seen by anyone but you, is still copyright infringement
That's the important bit - the things you called illegal a few comments back were obvious examples of (presumptive) fair use.
More generally, whether something is copyright infringement can be vague and subjective - so the bar for going around saying "yes, that is infringement" is not "does it meet a description I read online?". The bar is: "have courts previously ruled that a very similar case was infringement?".