I think it works the same as the Kindle itself: You can login and download the book as many times as you need [assuming they may set some limit for theft/abuse protection]. I'll try to verify that the next time I log onto my account.
I don't think there is any way to transfer files as they are locked up by the Kindle software. Also, on Tsu's newest Kindle, the memory card is no longer removable. The first generation allowed chip swaps.