To delete a Part or Volume, hover over its title in the left navigation pane and click the 'X' icon next to it. Atticus will ask you if you want to “Keep Children” or “Delete Children”. You must select "Keep Children" to not accidentally delete all your chapters within your part.
If you would like to delete all the pages and chapters currently nested in the segment, choose Delete Children. If you would like to keep the content, choose Keep Children and Atticus will place them back in the normal organization of your book.