This Handbook is intended to allow the reader to be able to learn the Mediawiki software well enough to make tweaks in the core software (or add extensions) which can either be added (committed) to the main branch of the code (if accepted by the development team) or simply used behind the scenes in one's own code.
It seeks to provide a more detailed guide to its structure than the overview file (help.doc) which is included with the software provides and discuss variables, etc. not covered in the PHPDocumentator automated documentation available elsewhere online (I don't see any way that it lists all variables used--that would be a nice enhancement to the Documentor software itself). There is likely much relevant documentation already at Mediawiki sites (e.g., at http://meta.wikimedia.org/ ) but none which really can provide a tutorial for learning how to follow the flow of the code in detail (and to use this to enhance, extend, etc. the software).