Reference documentation for Draw2d and GEF is provided in form of the GEF and Draw2d Plug-in Developer Guide, which can be found in the Eclipse Help. It contains a Programmer's Guide as well as the API Reference (Javadoc) for Draw2d and GEF. Reference documentation for Zest is currently not provided. There are also several Articles, Tutorials and Slides that explain how Draw2d, GEF, and Zest work.
The following material provides additional documentation and may serve as the basis for building up (further) reference documentation.