Apple

Depending on what kind of router/network setup you have, you might be able to block books.apple.com and you'll never see another ad or 'suggestion' in the Books app again (usually it's considered a parental control feature). To disable the ads when you're off WiFi, go into the settings app and disable the Books app's ability to use cellular data.

That is what I'm doing and it works great for now, but I'm in the process of exploring third party Book readers. The AppStore is such a garbage filled wasteland though that I'm not hopeful.