Sidney returns Juliet's letter and assures her that given what Susan said about the success of the book launch luncheon, Juliet's upcoming book tour will be wonderful. He does suggest that she not throw the book at the audience, like she did once when she was a teen. Sidney explains that while Sophie wants the tour extended to Scotland so she can see Juliet, he's doing his best to be vague and infuriating. He says that the sales from Izzy Bickerstaff Goes to War are excellent, and because of this, Juliet shouldn't worry about not wanting to write English Foibles. He asks that they get dinner before the book tour.
Sidney's suggestion about not throwing books at the audience indicates that Juliet has always been a bit wild and prone to actions that demonstrate her independence. In other words, Juliet's desire to become independent from the Izzy pseudonym isn't anything new; it seems a perfectly normal and expected result of a lifetime of independent thinking and an attempt to write with Juliet's true voice.