On the one end - somebody writing a book alone with a pen and paper, about an idea they had in a dream. Whilst they didn't make the book from nothing (somebody else created the pen and paper, somebody taught them how to write and structure a story, they are just using them to write a book) This is very clearly the work of that one person.

On the other extreme, imagine a wealthy person hires a writer to write a book. They specify that it should be about unicorns but nothing more. Let's imagine that they haven't even read the book yet. Can the wealthy person claim "I wrote this"? Most people would say no. They caused the book to be written, but didn't actually write it.

In my mind, asking an LLM to create something for you is more like the second example than the first, but there isn't a hard line between those two. I can think of plenty of examples which sit between the extremes (spellcheck, proof-readers, collaborative projects, re-tellings of existing stories). LLMs are in this space. You could have an idea for the book and lay out notes and chapter outlines and more, and have the LLM bring it together. Did you write the book? Who can say? Investigate it hard enough and the question becomes meaningless.