Books disappeared after importing an ePub document into the Books app on my iPhone 7 Plus

I have an iPhone 7+ on iOS 15.8.2. I was importing an epub and clicked on the “Importing 1 of 1” bit in iBooks which then shows an area of things Uploaded to iCloud. Next thing I know when I am back on the library screen, all my books are gone except super old ones synced from my other iPhone and only the book I had just imported at that time.

Luckily, I backed up my phone to my PC yesterday but when I tried to restore, it claims not enough storage to restore which I don’t fully get as I haven’t wasted that much extra storage in 24 hours.

Is there a way to get back the books without restoring the backup or must I have accidentally deleted them?

UPDATE: Any new book I now download and import to iBooks also disappears after successfully importing. And if I open it, iBooks clearly remembers what page I used to be on before this book somehow disappeared.

[Re-Titled by Moderator]