“Mercurial: The Definitive Guide”, by Bryan O'Sullivan, is a freely licensed book about the Mercurial revision control system.
Home page: http://hgbook.red-bean.com/
The HTML version is split into one web page per chapter.
The source code is available to the adventurous.
Translations
Chinese Translation
Source code (Work in progress...)
The HTML/PDF/eBook (Chinese & English) version is available.
See also: BuildingMercurialBook (OUT OF DATE, PLEASE WAITING FOR UPDATE)