Rating: 2/4 (Average on Mercurial content and usage)
My interest in Mercurial was piqued after reading Hg Init, an excellent introduction to this Distributed Version Control System (DVCS) written by Joel Spolsky. So, after installing Mercurial and TortoiseHg, I picked up Mercurial: The Definitive Guide to learn more about this DVCS. Written by Bryan O'Sullivan with help from contributors on the Internet, an updated version of this book can be found online.
The book is an easy quick read, considering it is technical writing. This is helped a lot by Mercurial itself, which is a much easier DVCS to grok than its competitor Git. I found the first half of the book which deals with introduction, daily use and common operations quite useful. The chapter Behind The Scenes which describes the inner workings of Mercurial is both hard to understand, mostly pointless and placed too early into the book. The latter half of the book which deals with Queues were also mostly irrelevant for me. I would have preferred more chapters which dealt with typical workflows in branching and merging.
On the whole this book is a good resource, especially since it is also available online for reference. Right now, this is the only deadtree book available on Mercurial. Considering the maturity of Mercurial and the average quality of this book, there is definitely an urgent need for better written books on Mercurial. Apress seems to have an upcoming Mercurial book titled Beginning Mercurial. I will be checking it out once it is available.