Cover page can not be displayed

I found an easy solution...

However, it requires macOS High Sierra. It may work with macOS Mojave, too.

1. In the Terminal copy and paste... open ~/Library/Mobile\ Documents/iCloud\~com\~apple\~iBooks/Documents/
2. Create a new folder on the Desktop.
3. Drag all your books and PDF documents from the iCloud Books folder to the new folder.
4. Wait for all your books and PDF files to disappear from your iPhone.
5. Drag all your books and PDF documents from the new folder back to the iCloud Books folder.

If iCloud doesn't begin uploading, try restarting your Mac. Patience is key... it may take a while for all your books to reappear. When they do, each one will have a book cover.

Worked for me too. Catalina on my iMac and iOS13 on my iPhone 6s and 1st Gen iPad Pro. Make sure you delete the existing files on the OSX device, and reboot the machine before you open them on the iOS device.

You can just share the epub files from your macbook to your iphone via airdrop, then choose 'Books' as opening option. This works for me and saves the uploading and downloading trouble of the icloud method.

Re-uploaded the book and tried again this worked perfectly!

Thank you guys for the solution (remove the problematic books from the cloud, save originals on the Mac and copy via airdrop to iPad). But, to add something to the discussion about Apple and their motivation to solve this problem: the principal issue I had was that the books I made in iBooks Author missed the cover art on the iPad (they were showing on iPhone en Mac). So, here we are talking about books that are made with Apple software and are meant to be used on iPad that don't sync as they should. No external parties or interests, just Apple. So where is the solution, dear Apple?

The easiest methods that i found is send it through Airdrop and open in Book. It works well!

In the end, this wouldn't work for me. The files in the iCloud folder would open the Books app, but not copy into the library. The workaround I found was to load the epub files into Dropbox and try it that way. For some reason, that worked perfectly, and the books populated my Mac library. A major pain in the neck, but better than having those dire auto-generated covers. So thanks for pointing me in the right direction.

Update. After a restart the covers look good on my Mac too. It works.

All the "fixes" being proposed here only work if your library is tiny.

I have over 10 000 pdfs and epubs combined. Airdropping or moving them to other directories to import them one by one is absolutely not an option. This is a FUNDAMENTAL issue with Apple Books. Apple will neither respond to any of this nor will they ever bother to fix it. They want us to buy from their ecosystem. Sideloading books from other sources doesn't give them any revenue, so we are quite out of luck. If you doubt this, look at the mess they've made with iTunes and the "new" music app.

Everything in it is geared towards streaming from THEIR services. As much as it pains me, I believe it's time to get off iTunes Match. Dump Apple Books and iCloud Drive. It's time for new non Apple solutions. I really hate to have to go that route but they are pushing us all in a direction that I just don't want to go.

This Terminal thing doesn't work for me (Catalina, iPhone X). Only option cut from iBooks to iCloud folder, open on iOS device.

Hope apple resolves this soon.

This is a hard nope. I’ve got 2000 + documents. Not going to tap on each one to get a cover to show. I wish I had that kind of time and patience.

There has to be a better, less hacky way to fix this.

Don't misunderstand me. I am NOT saying they did this on purpose. I do believe it's a bug. But it's been around for so long that it's easy to see that they ARE NOT in any hurry to address it. And as I've said before, there is no real incentive to do so. Except for a handful of us whining about it. Hardly a priority...