Contents of Books disappeared after restore

Hi,

I am migrating from one Ipad MIni4 to another with more storage. I have done all the setup etc and everything is fine. I did a backup of the old Ipad and restored it to the new Ipad. Everything looked fine.

But, when I opened the Books app I saw my "Reading Now" books OK. I then switched to Library and it was empty. All 296 books and PDFs were gone.

When I went back to "Reading Now" those books had disappeared too. My books were obviously there a moment ago and now gone. All the many PDFs are scientific papers downloaded from various journals.

Where have they gone?

Many Thanks,

DC