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