Was "It's the real thing" trademarked? And furthermore, trademarks are only usually enforced among competitors. Apple Computer can't sue Apple Dry Cleaning for trademark infringement. Well they can, and occasionally do, but it isn't a given that they will win. Usually you have to show damages, and Apple Computer isn't damaged in any way by having a dry cleaner use the name of a popular fruit. Similarly, I don't think Coca-Cola lost anything by having the phrase "it's the real thing" associated with a literary work.
My question for the lawyers: Does a letter like this count as "defending a trademark"? Or would Coca-Cola have had to actually take these publishers to court over it?