It’s doubtful anyone can provide more help without getting into details. It’s less a matter of yaml configuration, and more about how you structure the content to serve the intention.
How would you describe the desired outcome and what exactly does each “site” contain? Are you aiming to use some specific theme you like? Are you developing one or your own? Are they going to be in a single languages, or many? How do you define immersion specifically: is it a matter of embedding multiple media formats; is the graphic design and theme matching the story; is it going to be a hypertext format with cross-annotations and dynamic indexes?
Assuming that a “series of sites” doesn’t mean that each “website” lives on its own domain, then what you’re looking for in its simplest form is a different section (in practical terms folders of files) for each book. Then again assuming each book’s site contains the content instead of marketing material only, you likely want to separate chapters into separate markdown files (“posts” that any theme displays) or as branch bundles (more files that the theme needs to display).
There’s also several Markdown to PDF & website toolchains, such as Getty’s Quire (which at least some years ago did use Hugo in the backend); BookPub; LeanPub; GitBook (pivoted to technical documentation books several years ago) and more. Of course people have been developing bookish and book themes for Hugo, but each theme has its traits and quirks.
Maybe you’d want to get started with the content first, see the previous toolchains for tips on structure and format, then you can transform this content around into a website at a later point. Unless you’re already there.