Welcome to GitHub Pages.
This web site owns the sources as explained in the different chapters in the book. You can either get the compressed files in zip or tar.gz formats but the best is to use the git tools.
Git Tools
Clicking on the "View project on GitHub" button brings you to the project's page where all the sources can be read and downloaded. Anyway, as said you can install the github tools and clone all the project locally :
$ git clone https://github.com/dlewin/BeagleboneBlack-Server-Book.git
you can either use the GitHub desktop tools or directly clone to your board