Thanks for the response.
The books on my iPAD have not been backed-up in iCLOUD or in iTUNES. There is no iTUNES app in the desktop, as far as I can see. There is a MUSIC app, however. With it, there was no option for me to back-up the books or files to my computer from the iPAD. When I "choose restore" my iPAD, I get a warning that only the settings, contacts, etc. will be restored. Files and books are not backed-up, as far as I can tell. I did the "restore" and the iPAD re-booted as expected. Photos and music, all still there. However, again no books and PDFs.
When I open the MUSIC app with my iPAD connected to the computer, there are no books showing. Only a couple of documents. Alternatively, when I click on the iPAD in the "Locations" portion of the Finder Sidebar, there are 10 tabs across the top. I can view the contents under each tab. The photos show on the Photos tab. Music shows up on the Music tab. Books show up with the Books tab . . . BUT these are "greyed out" and I cannot do anything except view the list. 16GB are showing as used for Documents & Data (which, I presume includes the books and documents). All of the books and PDF originally transferred to the iPAD came from other sources, so cannot be retrieved from the Apple Bookstore.
So . . . I am stumped.
Up until yesterday, everything was just fine. IPAD II with 9.3.5. I realize this iPAD is a dinosaur - and the iOS cannot be updated further - but there was no issue at all with the books which I read yesterday. Only when I turned on the device this morning, the books and PDFs could not be found using the iBOOKS app.
It is not the end of the world because the files and books are on the computer in the folders where these were originally downloaded/created. If I want to go to the trouble, I could transfer these again from the computer to the iPAD. I just am just puzzled as to where the books/PDFs went from last use last night to first use this morning . . . and if there is a way to re-access on the iPAD without having to transfer these items again from the computer.
