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).
[1] One example: https://boston.conman.org/2002/12/31.1
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.
Sometimes ads end up in the actual text, without the authors permission or even knowledge: https://www.reddit.com/r/Fantasy/comments/q1wbie/the_time_te...
https://lithub.com/the-time-terry-pratchetts-german-publishe...