Documentation is here:
Amazon Kindle Development Kit.
IMO, the best chance to extend the Kindle without the KDK, while keeping the Amazon features, is to grab an older ROM that isn't obfuscated, dump it, run JAD on the classes, and use it and a combination of the KDK docs to piece together a new booklet (application) that can run on the Kindle alongside the existing framework. IIRC someone already made a very simple booklet this way.