This document contains both the libwps API specification and the normal libwps documentation.
The external libwps API is provided by the WPSDocument class. This class, combined with the librevenge's librevenge::RVNGTextInterface class, are the only two classes that will be of interest for the application programmer using libwps.
If you are interrested in the structure of libwps itself, this whole document would be a good starting point for exploring the interals of libwps. Mind that this document is a work-in-progress, and will most likely not cover libwps for the full 100%.