How do I activate H5P for my book?

Activate the H5P plugin by going to Plugins from the left sidebar menu while on your book's dashboard in Pressbooks. Find "H5P" in the Plugins list and click Activate. 

Afterward, a new menu item called H5P Content will appear in the left sidebar. You can make new H5P here, or adjust your H5P settings by going to Settings > H5P.