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boskyout: downloading and reading the GPL source code from amazon is a good place to start. It is written in C.
You can also study the scripts in the /etc/ folders in your kindle.
There is also some javascript code, and HTML5, and various configuration files inside your kindle that are worthy of study, depending on what interests you and what kind of projects you want to work on.
Another thing to examine is the source code and scripting distributed with most kindle hacks.
And there are some kindlets available as well, written in Java.
Another rich source of valuable information is the IRC channel archive:
http://kindle-synchrone.dotcloud.com
And of course, there's the wiki page:
https://wiki.mobileread.com/wiki/Kindle_Touch_Hacking
It looks like you have a lot of reading ahead of you.