A few NaNoGenMo books have made it into print either through self-publishing platforms like Lulu or Blurb or (usually in different form) via small press publishers like Counterpath, Bad Quarto, or Aleator.
The requirements of NaNoGenMo are format-agnostic (and many are plain text or Markdown), but conventionally, I prefer to think of a PDF as the ideal format because it is designed to invoke the formal qualities of a book and at least gestures toward immutability and individuality
As Lisa Gitelman writes
PDfs are individually bounded and distinct ... already authored entities, understood as distinct from the written systems in and by which they are individually named and potentially manipulated or downloaded. (Gitelman, Paper Knowledge, 2014)
The Colab Notebook linked below provides a method for generating a book-like PDF using libraries available or installable within the Colab environment.
PDF Workflow with Markdown and WeasyPrint