I've found books that had ads inserted into them [1]. It seemed to be a thing from maybe the 1960/1970s. The ad page was a different type of paper, and no text from the book was on it (that is---the ad wasn't on one side and book text on the other).
Paper magazines still have "blow ins", though - advertising cards that are injected into the magazine with compressed air after printing. They're not bound in. They fall out.
There was a post, here, some time ago, about how many paperbacks had ads actually woven into the story. Apparently, it was quite common practice, at one time. Sort of an obnoxious “product placement” thing. I think the author had nothing to do with it.
[1] One example: https://boston.conman.org/2002/12/31.1