It can be done already, but the quality of the book will always be in question. You just break down what ChatGPT is doing into small consumable tasks. You don’t say, “Write me a book.” You say, “Please give me an outline for a book and list 12 chapters with titles relevant to the plot. Then you ask it to write each chapter. I’d recommend you also edit as you go. ChatGPT is a tool to save labor, but you have to give it reasonable, consumable inputs versus your expectations.