Packt Publishing has made an announcement about a new book that's just been released - "MediaWiki Skins Design" (by Richard Carter).
From their release notes:
This book takes users through customizing their MediaWiki skin. It is full of practical examples of MediaWiki skinning techniques, and provides a clear explanation of how MediaWiki skinning works and the necessary template and CSS customizations required to completely alter MediaWiki's appearance.
The book has appeal for both web developers and wiki administrators by helping them update the look and feel of their MediaWiki site. You can find more information on the book on its page on the Packt Publishing website.