While Atticus doesn't allow you to combine entire books, outside of creating a box set, we do allow you to import chapters into one of your books. If you need to add some content into a book that you’ve already started in Atticus, you can save that chapter or group of chapters in a new .docx file from Atticus and import that into the book of your choice. You can learn more about how to do that here: Can I import chapters into an existing book?
Hope this helps!