The book introduces Mercurial to the reader. It explains branches and why it may be better to use local clones instead. The author clearly likes the tool set he writes about, but the book does not end up in blind love to what is just a tool after all. He also mentions its glitches and problems, not just its strengths and in the later chapters tells us how to customize Mercurial for our needs through the power of Python.
Though the book is available on the internet free of charge, I prefer the printed form. But I encourage you to have a look there first and buy the book if you like it and don't want to rely on someone else's review.