How can I permanently delete iBook books stored on iCloud?

At least in the USA consumer law doesn‘t allow Apple to modify your purchase history (no online retailer of 3rd party intellectual property can do so). When you purchase an iBook you purchase DRM rights to that from the publisher, not Apple. Those DRM rights are yours forever, and the only proof of that is in your online iBook store purchase history. Apple is required to keep that record since they don’t own the DRM rights sold to you, they just facilited the sale, and they serve as the warehouse for access to that DRM protected content. Same thing goes for Apps - you purchase a license to use. That license is issued by the App developer, not Apple. Apple is just the retail agent and warehouse keeper of acces to the licensed software.

Just as Microsoft cannot edit or alter, or allow any altering by you of your purchase history with their online software store or their XBox store, or Sony for their PS4 online store, or Barnes and Noble with your online purchases, etc, etc. They are prohibited from doing so.

Just like iTunes and App Store purchases, you cannot delete iBook purchases from you purchase history. If they are not on the device and showing the download cloud icon, that just means they are available from your purchase history in the iBook store.

If you use iCloud (your personal online account, and distinct from Apple’s cloud-based online store fronts) to sync iBooks across devices, then just be sure when you delete the iBook you delete the download from all devices, not just the local device.

That is something to submit to Apple feedback - Product Feedback - Apple

It is nice that you can hide app purchases, so suggest that same thing to Apple about iBooks if you’d like to see it.

For a long time iBooks were not available for re-download (like Apps and iTunes stuff has always been) so it is a bit behind the curve relative to some of the things you can do with the App Store. I really wish they’d add more refined categories to browse books too - “fiction”, “mystery” and “sci-fi” is a pretty minimal breakdown of recreational reading material 😁

Hi. If you're just seeing them in the iBooks app, with the cloud icon alongside, I don't think they're actually stored in iCloud. iBooks just remembers that you "purchased" them and could download them again. That said, I don't think you can delete them so iBooks forgets them. You could create a folder and move them to it, so they're out of the way.