The book metaphor - structuring the contents
The contents is hierarchically structured, like a book, in sections, chapters, sub-chapters etc. Introductory pages into the contents sections offer an overview; more in-depth information is offered in corresponding units further down the hierarchy.
The respective position within this structure is represented by a navigational element at the top right in the header. Links on the same or lower levels can be reached via the navigational element at the bottom right (bottom left on overview pages).
Obviously, there are also a lot of cross references in the text itself, which can easily be followed on by mouse click.