I've been buying books for decades, and I remember when the plague of slick, stiff paper ads inserted in the middle of paperbacks fell upon the book buying public. Usually cigarette ads, as I recall, probably because that was after cigarette ads were forbidden on television and radio. They were annoying as hell because they tended to make the book open to that point by default--and you couldn't cleanly pull them out, either.

Fortunately, this plague on book buyers was fairly temporary, probably because it wasn't a good investment for the advertisers. A given book's ad can only reliably be expected to be read by ONE reader, and there was no way of tracking who it was, whether it influenced their buying habits, etc.