About Mercurial: The Definitive Guide.
1. Reading the book
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/
An HTML version is split into one web page per chapter
The source code is available to the adventurous.
The epub version for ebook reading devices.
2. Supplemental information
The book covers Mercurial through version 1.0, which predates a number of important features that you can read about here:
The summary command
The resolve command
The rebase extension