Why are my Apple Books documents disappearing?

Re: Has anyone heard or seen a definitive explanation from Apple as to why documents disappear from (i)Books?

Wondering if your iOS device is short on memory / storage space ?

(If you use iCloud: iPhone storage space is quite different from how much iCloud space you have available.)

If needed: see: How to check the storage on your iPhone and iPad - Apple Support

Eg: An iPhone generally needs around 8 Gb free in which to operate smoothly, and do all you ask it to do.

If memory / storage space is allowed to become too low:

Seems items that can easily be redownloaded may sometimes need to be removed in order to to keep a device working as expected, or to complete an update process. (Some deleted items may remain available in the "Recently Deleted" folder, and if so, can generally be restored when space permits)

Else: If needed, see how to: Redownload books and audiobooks - Apple Support

Thank you kindly for your reply, and for checking current memory / storage.

Apple Support have an article online that may be helpful: If you don’t see a book in your Books library, or your books and PDFs aren’t syncing across devices - Apple Support