In a textbook kind of book, new concepts are introduced to the reader. In a book with hyperlinks, such as an e-book or a book on the web, it is possible to link the introduction of a concept to a table with all concepts in the book, bundled together with explanations, and perhaps also with links to be used for further reading.
In a Book with Views, this can be extended to a per-view handling of the concepts. You can see an example of this, in the introduction of the concepts variable and value in the beginning of the chapter called Names and values in the book Into Programming.
The collection of concepts can also be reached via the book’s About page.