Many years ago, I puzzled over why books didn’t have ads (here and here). The two best explanations were (i) that ads needed to be refreshed and in books would become old and (ii) that they didn’t suit the medium. I still think the latter is most relevant but news today that Amazon is working on the former.
You know what my favorite part of Moby Dick was? The part where I realized I wanted a Klondike bar. That experience may be expedited in the future, courtesy of an Amazon patent for a technology which would dynamically insert ads into e-books. And get this: the ads would be context sensitive. “For instance, if a restaurant is described on page 12, [then the advertising page], either on page 11 or page 13, may include advertisements about restaurants, wine, food, etc., which are related to restaurants and dining.”
So long as it is clear what is going on I guess there is no problem with all that. I can’t imagine the ads being effective but I certainly think it is legitimate to try.