...who holds the copyright to the book you are writing?
This is not as simple a question as it sounds. Before digital distribution, owning the copyright on a book in a world with no publishers would have been pointless. So while a writer might be granted a copyright, it was nonetheless for the benefit of distributors. People don't need to be financially incentivized to create art, but they do need to be financially incentivized to invest in printing presses, shipping networks, and book stores.
This is not as simple a question as it sounds. Before digital distribution, owning the copyright on a book in a world with no publishers would have been pointless. So while a writer might be granted a copyright, it was nonetheless for the benefit of distributors. People don't need to be financially incentivized to create art, but they do need to be financially incentivized to invest in printing presses, shipping networks, and book stores.