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.

My dad has some old sci-fi books with full color cigarette ads in the middle. Crazy!

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...

That was so unpopular that it died out.

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.