Monorepo vs. Multirepo

With multirepo, each book/project gets it's own repository. With monorepo, a single repository contains all content for all books and projects.

Multirepo Advantages

Each book/project is independent of changes made to other books. If a piece of similar code is corrected in another book or project, you don't need to take the fix in a different book until you're ready to address it.
Book URL: https://raytracing.github.io/InOneWeekend/

Shared resources such as release tools, book CSS, Markdown scripts, project documents and so forth have a single source, and need not be copied+added+committed+pushed+PR'd to multiple locations.
To get the entire suite, you only need to clone a single repo, or download a single package.
The project gets a clear common version number.
Getting the latest versions is accomplished with a single git pull, rather than having to go to multiple clones to pull from each.
Releases are easier, as they include all content, rather than needing to get a release per book. This includes the work of a detailed and useful CHANGELOG, as changes may impact multiple books.
Issues that impact multiple books are much easier to address. Multirepo requires filing similar issues in each impacted repo.
The entire project can have a single common wiki. This is a weak advantage, as we could also just direct each book to the a common wiki under the raytracing.github.io wiki.
Book URL: https://raytracing.github.io/InOneWeekend/ (if monorepo is raytracing.github.io, subdir InOneWeekend)
Book URL: https://raytracing.github.io/RayTracing/InOneWeekend (if monorepo is RayTracing)