iCloud Books: Failed to Load Book Because the Requested Resource is Missing

Since updating to iOS 9.3 and 10.11.4, I've decided to finally try and get my eBooks all fixed up and synced between my devices, since I swap between my iPhone and iPad often when reading.

I'm using ePubs. They work fine on my Mac. When I download them directly to a device through "Copy to iBooks" through mail or through iTunes, they read fine on the iPad and the iPhone.

However, when I activate the new iCloud Books, half the time the book opens on the cover with the spinning gear. After a short time (10 seconds or so), I get the error: "Failed to Load Book Because the Requested Resource is Missing."

I am pulling out my hair trying to figure this out. I've run the ePubs through the Calibre ePub covert and even edited them in Sigil. The ePubs are perfect in direct download to the device, but about 85% of the time, they give that error whenever downloaded from iCloud Books. I've compared the ones that work to the ones that don't and can't find any differences.

I'm totally lost. Can someone PLEASE tell me what the hell I'm doing wrong?

The most likely reason is that you were using different versions of the same book, or two different books are name the same, something like that. The Apple Books app remembers the position for one version of the book, but can't find it in the new version, and pops up the error.

Source: http://richardexplainseverything.blogspot.com/2016/05/fixing-failed-to-load-book-because.html