They are interactive. What AI is doing with story generation is a text version of the holodeck, not just a plain old book. You can interact with the story, change its direction, explore characters and locations beyond what is provided by just a linear text. And of course you can create stories instantly about absolutely anything you want. You just throw some random ingredients at the AI and it will cook a coherent story out of them. Throw in some image generation and it'll provide you pictures of characters and locations as well. The possibilities are quite endless here. This goes way beyond just generating plain old static books.