Links are pretty straightforward. For instance, you might link to the Electric Book template.

In print, that link shows in brackets after the linked text.

Buttons should be defined by adding a .button class to any element, such as a link. The template allows for and block-level buttons:

Again, the print version should have a way to deal with these. Alternatively, the book author can hide buttons in the print version. For instance, themes may support adding a .non-printing class to the element. Or the author can use Liquid control-flow tags, like {% if site.output == "print-pdf" %}, to control whether and how buttons appear in print.