Any OS induced (with or without USER help

) errors would show as an error.
Do you download NEWS? There is a feature that removes older entries with the tag NEWS ((I think the option of Fetch News is 'Delete after n days" ), but that might have snagged your books if they had that tag.
The only other thing would be incorrect usage of the Delete Formats (for selected)
ALWAYS review the
STATUS LINE at the bottom. it has a vale for Selected. It should match your expectations