This is a great question! While we don’t have the option for you to split your book while working on it in Atticus, we do have workaround options for you to accomplish the same goal:
Option 1:
Duplicate your book
Delete the first few chapters and keep the chapters you’d like in this book
Option 2:
Export a docx of your manuscript from “Edit Book Details”
Split your docx in Word
Import your new docx as a new book into Atticus
We hope this helps!