AI is a distraction here. You can use whatever tool you want. Copyright does not care how you made it. It cares about likeness and in imagery, music and text, especially names.
I’d assume you can freely use content from the earliest works, so if you eg use a character introduced in a later book it would still be covered by copyright. But I’m not sure.
I’d assume you can freely use content from the earliest works, so if you eg use a character introduced in a later book it would still be covered by copyright. But I’m not sure.