This question is asked at least monthly. And I'm always a little baffled as to why. The books on the device are the ones you put there. You can pick it up and look at any time. Or, connect the device and let calibre tell you.
But, maybe it is just me. I decided a long time ago it wasn't worth tracking. Instead, I track what I am going to put on the device. I have a Reading List for each devices. When I get a new book or change one, I add it to the lists for the devices I want it on. Plug in the device and the books are sent to it and the list is cleared. Hence the devices have the books on them that I want on them, plus the ones that will be sent the next time. And if I add a book to the list that is already on the device, it too will be sent with no damage.
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