At this time, Atticus doesn't support having multiple/different running headers for different sections of the same book. The header and footer, as all theme settings, are global settings that are applied to your entire book. However, we do have a workaround as we know it's important to be able to add a preview of your next book.
Hide Headers on Specific Chapters
The best way to ensure that you don't have the incorrect header on your new book is to hide the header and footer for those specific chapters:
Open the new chapters in the Writing tab.
Click on the cog icon at the top of the chapter.
Select "Hide header/footer" to remove the running header from your first book. See screenshot below for reference.
We hope this helps!