I think book pages here on sitepoint report the book latest update and publication date in a confusing manner. Sometimes the publication date is updated with the "latest update" to the book dtae and i think it is confusing. I think you should keep the publication date in line with the initial publication date of that edition of a particular book and then keep two different "latest update" dates, one for the pdf version and one for the printed version, as the two version of the book are updated separately.