There are now three Kindles in my family. The original one, the 2nd Gen, and now the DX of infamy.
I’ve bought lots of for-fee books from Amazon for each one, converted a lot of Gutenberg-esque free texts for them, and “purchased” a bunch of the free books that Amazon offers as well. Plus a half-year of subscribing to a fiction magazine.
Lots of content, all scattered about. Turns out, more of it is capable of crossing devices than I would have suspected. As someone who works with cryptography for a living, I always wondered about the security implied in the service Amazon supplies, to convert your personal documents into the kindle format. I need not have worried on their behalf; they don’t encrypt your personal documents, only re-encapsulate them. They give you a different target address for each device, even when you’ll just be downloading to PC, so I assumed they were doing some sort of differentiation in content sent back. But when trying to play around with Kindle for PC, I had to set up a separate Amazon account, I discovered that not only were all the personal documents, regardless of target machine, openable by the different account-ed PC, so were all the free-from-amazon public domain books, and the magazine issues.
So, it turns out you can share kindle docs with other users, just not ones you paid for. Seems oddly reasonable.