Publishers/authors could curate an entire cloud of hashtags for a book that would change as the discussions about the book evolve. Some tags would die, others thrive and subdivide. The cloud would contain them all.
How could you maintain this flexible ecosystem of tags with thousands of “static” pbooks in print? A single QR code that links to the updated hashtag cloud for the book. This could be as simple as a site that allows users to sort all the tags in the cloud – by date, popularity, location, etc. – and then jump off into the various discussions as they wish. (An offshoot/startup idea: a search engine designed to search and sort hashtags and their clouds.)
With QR codes, your existing pbook or print campaign would always give the customer a way into the most current iteration of all the relevant hashtags that exist for the book – some author- or publisher-generated, some user-generated.