Tap New on the Books list, or choose Books from Create, to start a book. It starts as a private draft owned by your account.
A book's Contents now show front matter, chapters, back matter and set-aside sections under their own headings. From a section's menu you can move it up or down, move it to another group, or set it aside. If the contents changed somewhere else first, the app reloads them rather than applying the move to an order you have not seen.