Haiku's API reference is currently very incomplete. We can't reuse the Be Book from BeOs, as this one has a too restrictive licence.

Go to http://api.haiku-os.org , and find something that isn't documented.

Pick the source code of the class you chose, for example at http://dev.haiku-os.org (you can get the source from svn or use the web browser).

Write some text about what each method in the class do. Look at the existing documentation for other classes to get an idea of what to say.