While setting up story prompts for the Author(s) to write about, the Purchaser or an Editor can sort upcoming unsent story prompts by clicking and dragging them in to the right order.

But what about once all the stories have been written? How can the order of those stories be changed for the printed book?

That's easy as well. The Purchaser has the ability to do that once the subscription duration for sending out new story prompts has ended, or if before then the Purchaser decides it's time to print the book. At that point the Publish Book Task List appears. The first item in that list is Approve Story Order. Clicking Approve Story Order brings up a window where story prompts can be drag-and-dropped into the correct order. Clicking the Save Order button at the bottom of the window saves those changes for the book.