Affinity Publisher facilitates the creation of documents of all sizes and types but version 2 has a dedicated feature to underpin the creation of Books.
The key difference with the Books feature is that it allows the final work to be assembled from multiple separate files. To appreciate the huge productivity boost that provides you need to understand what is wrong with the single document approach.
A Single Tome?
Creating a book in a single file means that file has to contain everything:
The Cover
The Front Matter
The Table of Contents
Multiple Chapters
The Index
The Back Matter
That's a lot of material to navigate, edit, update and generally manage in a single file. Cognitive overload for you and potentially stressing the processing power of your device.
Separate "Books"
The Book feature enables you to create separate files for each chapter or section of your work and work with them in isolation from the rest of the content. This means:
Content management overhead is reduced
Collaboration between multiple authors is simple
Consistency of styles and formatting is managed centrally
The Books Panel
File > New Book displays the Book Panel where you are able to create new Books and add new "Chapters" to the Book from existing files. It is also where you control the ordering of chapters, remove or replace chapters and synchronize content including Swatches, Text Styles, Table Formats and Master Pages.