Tip, especially for the new contributors:
MediaWiki Virtual Library
http://www.mediawiki.org/wiki/MVL
has currently three books
most important the MediaWiki Developer's Guide
http://www.mediawiki.org/wiki/MDG
Using the Extension:Collection ("book") function, you can always
generate your own up-to-date copy of the essentials, or even let a book
be printed.
Tom