Addition to post #105
It seems that if only one book is added at a time, the chance of causing a (disappearing books) problem is reduced. However, it also seems that certain books cause the problem, too. I can not identify why.
(Add 1 book)
Add 1 book, it appears as "on device", eject device and check, Kindle screen shows 1 book more than before. OK
(Add 2 books)
Add 1 book, it appears as "on device", repeat, eject device and check, Kindle screen shows 2 books more than before. OK
(Add 3 books)
** Add 1 book, it appears as "on device", repeat, repeat, eject device and check, Kindle screen shows "My Items = zero".
Deleting those final 3, one at a time, did not fix the problem, until I deleted the final one of the offending 3.
Then noting what the 3rd book was, I added that alone. Problem occurred again. So, skipping this book, I added the other 2, as above. Those 2 books appeared on the Kindle screen.
For the time being, I will do this: Add 1 book, eject device, check Kindle screen, if good, next book, and if bad, remove that book and leave it on the PC.
Sorry if I have missed a point that someone has made that explains or fixes this.
I wish I could explain it more clearly in fewer words.
Still impressed with what Calibre can do for me.