Three levels, fixed

A book is an ordered structure of three levels:

  • A book is a whole body of documentation: a product manual, a training course.
  • A unit is a chapter within it.
  • A page is one article within a unit.

The depth does not change: a unit cannot hold another unit. That is a constraint, and it is the point: documentation cannot grow a private structure nobody else can find their way around.

Why not just use pages?

Pages nest to any depth and suit a website's general structure. Documentation needs reading order. Because a book knows the order of its units and each unit the order of its pages, the site can show a table of contents, previous and next links and page numbers without anyone maintaining them, and a book page, or a whole book, can be exported as a PDF.

What it costs

Flexibility. A book page always sits in a unit, and a unit always in a book. Content that does not belong to a chapter is probably a page, not a book page.

Choosing between them

Use a book when the content is meant to be worked through in order: onboarding, manuals, training. Use pages for content read on its own and reached from search or a link: news, marketing, policies, landing pages.

A useful test: if "what comes next?" is a meaningful question for the reader, it is a book.