As illustrated here by the Tru64 UNIX Documentation Overview, most of the books in this library are framed documents. Each framed book has a Table of Contents, a text panel, and a button panel. In the button panel are buttons to go to the book's index (if there is one) and to open this help window.
Here, the user scrolls to the bottom of the Table of Contents, goes to a table in Chapter 6, then uses the button panel to go to the Index and from there to some information in Chapter 4 about the man command.