I subscribed to the BookBub daily email so I could observe their ads. I'm subscribed to the Science Fiction, Thriller, and Young Adult emails. I've been served three different versions of ads for this book, and each time it was served four days in a row:

We know that BookBub serves the same person each ad four times before moving on to the next email addresses. For CPC that's okay, but for CPM it seems a waste. To me, it seems like 75% of the CPM ads are wasted because if the recipient didn't click the first time, they aren't going to click the 2-4th time either. I asked David Gaughgran about repetitive CPM serving and he insisted it's a good thing.

In any case, if that ad was simply swapping out images while running the same ad campaign, it would have skipped over me when serving the second and third versions because I would have already been targeted four times. But since I keep getting ads for a book I will never click on, that tells me the advertiser is starting a new ad each time with a new image. That also indicates that BookBub serves the ads from the beginning of their list of emails each time too, rather than randomizing the list.*
Which, it seems to me, means that if you keep starting over with new ads, those ads will start over at the beginning of the email list and be served to the same people who saw your earlier ad the first time. Which, theoretically, should drive your CTR to zero if you keep starting new ads for the same book and BookBub keeps starting them with the same initial targets. Or am I missing something here?
* It could also indicate that BookBub has a lot of excess capacity and ads like this are back-filling the open slots. Which would point to a low-bid strategy to see if your ads could run in those slots too.
ETA: Aaannd I just received today's BB email with this in the ad slot:

I'm glad I'm not paying for these ads!