Books are created and their contents managed using the Books panel, on which you can add, remove, replace and rearrange chapters.
Each document added to a book may represent a literal chapter of your publication but it doesn't have to. How your publication is divided into discrete documents is your decision, which may be influenced by your production workflow.
Multiple chapters can be simultaneously added or removed. One chapter at a time can be replaced or rearranged.
Replacing chapters can help in a collaborative workflow, e.g. upon receiving a version that has been copy-edited.
For books that contain facing-page spreads, there are two behaviours that determine how stray pages—single pages at the end or start of consecutive chapters—are handled.
Where one chapter ends with a left page and the next chapter starts on a right page, Affinity can merge these stray pages into a facing-pages spread.
Where only one stray page exists between consecutive chapters, Affinity can 'pad' the document, i.e. insert pages, in order to maintain facing-pages spreads.
Do one of the following: