Long story short (I hope):
I bought an ebook on Amazon, but it would NOT allow me to download it to my Kindle.
I contacted Amazon for help and this ended up in
de-registering and re-registering my Kindle.
It turned out the problem was with the ebook, something about the settings the author had for it. So I returned it and got a refund.
HOWEVER, when I connected my Kindle to Calibre -- NONE of the books I have on my Kindle (and there are at least 200+) showed as being on my device/Kindle -- even though I used Calibre to download all these ebooks to my Kindle. 
The "On Device" field is just *blank* for *all* of these books!
It would seem the de-registration/re-registration fouled *something* up -- but I do *not* know what this is or how to get Calibre to recognize all the ebooks I have on my Kindle as BEING ON my Kindle.
Does anyone know how to solve this problem? Please?
Thanks.
**