Hugo is mostly used to generate web sites. For on of my collections I reused a Hugo website to generate a book from it.
Have a look here:
It’s done with a set of custom templates (tied to a output format) to generate a large HTML file and using VivlioStyle (and thus CSS for paged media) for the layout (and other things like the TOC).
Yes, its completely automated: different sections from the web site are mapped to book sections. Posts are the main part, a special catalogue setting is responsible for the frame (like front page, preface, text contents, blank pages and so on). So publishing a new post will result in a new book.
There are no manual steps, if you just see the book as a simple sequece of pages. But a rela book is more then that, there are some rules like having a start of a chapter always on a right side. This might create a requirement of blank pages. then there is the requirement that a TOC should always fit a the page, this required adjustments to the font size.
Both things are certainly also possible automate, but that was to much effort for this project.