Drupal is an incredibly powerful CMS, but with that power, comes a lot of complexity. It is, in some ways, very different than most other CMS's out there, which adds to the learning curve. But many would say that it may be one of the strongest because of these differences... this is something each user will have to determine for themselves.
Who this book is not for... anyone who has a strong understanding of Drupal, perhaps already customizing Drupal.
Who this book is for... anyone starting out new with Drupal, struggling to gain an understanding, and certainly anyone new to CMS that has chosen Drupal or is considering Drupal.
I think previous posts did a good job of recapping some of the key sections, so I won't rehash those as they are pretty good and fair descriptions.
As the book even recommends, there is much information on the Drupal site, both online documentation and in the forums. So why buy a book? The online Drupal documentation is primarily a set of technical documents... not strictly dry technical, but still primarily focused on the "what." However, anyone just getting started will likely feel overwhelmed or just confused with trying to dig through that type of documentation. It is a must, but may be better off left for doing after reading this book. And some of us just like to page through a book and have a reference at our side, not to mention, reading tons of information online can be a bit harsh on the eyes after awhile.
At the time of this writing, to my knowledge, there is only one other book, "Building Online Communities with Drupal, phpBB, and WordPress" from Apress. As I have both, I can say that it is also an excellent book. The immediate thought may be "which one should I get?" Perhaps both, but either way, don't misjudge by size, as the Apress book combines three scripts into one book, so actual content length is fairly comparable between the two.
I would highly recommend this book as a starting point for anyone new. It provides a few more visuals with screenshots than the other book does, and does a great job walking you through a "hypothetical" example, much like a tutorial. The Apress book though would be an excellent next step, as it goes into a much deeper level, including multi-sites (running multiple sites off of one Drupal installation) which isn't even covered in this book, and more details on themeing. However, the Apress book might also be a bit overwhelming if you are really starting out at square one.
I had actually gone through the exact opposite approach... read the online documentation first, then the Apress book, then this book. The two books are laid out a little differently, and while there is obviously some overlap, Mercer and Douglass both have their own styles and presented information a little differently, so you may find that things you are confused on in one book may click in the other. That was certainly my case and I can only imagine that things would have clicked even faster had I read everything in the opposite order as I had.
If you've read the Apress book or any of the online docs, you'll still gain from Mercer's book, however I'd highly recommend starting here first if you can.
I would liked to have seen a little more information, and would really have liked to have seen more information on themeing and multi-site. These can be quite complex, but David does a nice job of presenting information so it would have been nice to see how he did on presenting these more complex topics. Both this and the Apress book are listed as Beginner-Intermediate, but I would consider this book to be more on the Beginner side and the Apress book to be more on the Intermediate side... again, making them a pretty good 1+2 combination.