I've been working on a Kindle Store proxy for the Kindle 4 (NonTouch), which works with Calibre. If set-up correctly (which may take some doing), it integrates (as seamlessly as it can) the list of books present in the Calibre library with the list of books the Amazon returns, allowing you to download books, from the Calibre library to the Kindle, as if they were on your Amazon account.
It is not feature-complete -- right now last-position-read, notes and highlights are not saved. I'll be implementing them as soon as I figure out the MBP format
The source code (KSP is written in Python 3) is available on GitHub
. The installation procedure and the changes that need to be made on the device are documented in detail; there are also some scripts to help with changing the device configuration.
I'm not sure how useful it may be to other people -- I've found it quite useful
. Right now it only works with the Kindle4NT because it's the only device I have to test it with; I'm guessing it could be easily deployed on a Kindle 3 (Keyboard) as well. Not so sure about Kindle Touch, as I understand it's quite different from the K4NT.
If you have any feedback, ideas or suggestions for further development, I'd love to hear them