Once there is code there will be parasites, viral, battening on the code. It's how life works: around 75% of known species are parasitic organisms. A large chunk of the human genome consists of endogenous retroviruses, viruses that have learned to propagate themselves by splicing themselves into our chromosomes and lazily allowing the host cells to replicate themselves whenever they divide. Spammers will discover book-to-book discussion threads just as flies flock to shit.
But then it gets worse. Much worse.
Authors, expecting a better reaction from the reading public than is perhaps justifiable in this age of plenty for all (and nothing for many) will eventually succumb to the urge to add malware to their ebooks in return for payment. The malware will target the readers' ebook libraries. The act of reading an infected text will spread the payload, which will use its access to spread advertising extracts and favourable reviews throughout the reader communities. You may find your good reputation name taken in vain by a second-rate pulp novel that posts stilted hagiographies of its authors other books on the discussion sites of every book you have ever commented on (and a few you haven't). Worse, the infested novels will invite free samples of all their friends to the party, downloading the complete works of their author just in case you feel like reading them. Works which will be replete with product placement and flashing animated banner ads, just in case you didn't get the message.
Finally, in extremis, feral spambooks will deploy probabilistic text generators seeded with the contents of your own ebook library to write a thousand vacuous and superficially attractive nuisance texts that at a distance resemble your preferred reading. They'll slide them into your ebook library disguised as free samples, with titles and author names that are random permutations of legitimate works, then sell advertising slots in these false texts to offshore spam marketplaces. And misanthropic failed authors in search of their due reward will buy the ad marquees from these exchanges, then use them to sell you books that explain how to become a bestselling author in only 72 hours.