I don't know if this is mentioned elsewhere, but the original "book" events were generated by the original Textbook functionality, which uses .png files.  In /lms/templates/staticbook.html one can find calls like:

  Logger.log("book", {"type":"gotopage","old":page,"new":n});

  Logger.log("book", {"type":"prevpage","new":page});

  Logger.log("book", {"type":"nextpage","new":page});

When I did the pdf textbook, I just generated the same events.  They could be differentiated by the url in the page attribute (with one containing "/book/" and the other containing "/pdfbook/"). That said, I did not put any logging events for HTML textbooks.