To learn the engine, our most important documents are:
Resources about the engine from the Web3D 2015 conference workshop:
More documentation about the engine:
Copyright Michalis Kamburelis and others. Particular thanks go to Paweł Wojciechowicz from Cat-astrophe Games for various graphics. Even this documentation is open-source and you can redistribute it on open-source terms, in particular CC BY-SA.