FeedHQ implements the Google Reader API—also known as the Open Reader API—to provide synchronization capabilities to client apps. This API was not officially supported and remained undocumented in the times of Google Reader. Finding extensive documentation about the API wasn't very easy as everything was scattered across wikis, blog posts and StackOverflow answers. Some good resources about that API even disappeared in the past year.
Having implemented the API, I know pretty well how it works. I decided to document it and publish the API reference on ReadTheDocs. It describes FeedHQ's API implementation but there are only minor differences with Google Reader's implementation. The goal still is to allow any Google Reader client to support FeedHQ with minimal changes.
I hope that FeedHQ's API docs will shed a bit of light and be useful to people who work with the Open Reader API in general!