Google Reader has no official API. However, folks have figured out ways of using the existing API even though Google has not officially published any documentation.
There is a Google Group: Friends of the Unofficial Google Reader API | Google Groups.
And there is a Python library where you can find lots of good API information: GoogleReaderAPI – pyrfeed
There is also a small PHP library: greader-library:
PHP class library for providing access to the Google Reader API. Enables authentication and provides public methods for general Google Reader actions.
I don’t think greader-library supports OAuth as of this writing, but this may change in the future.
Know of any other good resources for the Google Reader API? Let me know!