Thre is two ways of getting this book: the electronic one or the paper one. If you plan of using Mercurial, the paper may be better suited.
Mercurial (also called hg as the Mendeleiev symbol for mercurial) is one of the three DVCS (Distributed Version Control System) that are in the mood nowadays. Written in Python, its life started at the time as git’s when BitKepper was dumped as the Linux kernel’s VCS. Now it is a mature product, and the book tries to explain how to use it and also the differences with Git. Bazaar, the third DVCS, is not even mentionned, although it is also written in Python.