I've tried to give you a good idea about what this book is, but I should also make clear what this book isn't.
First, it isn't a troubleshooting book. Although in Chapter 14 I cover a number of maintenance tasks you can perform to keep your Mac humming along, and throughout the book I discuss a few of the most common issues users experience with OS X, the truth is that including substantial content dedicated to troubleshooting would have taken space away from topics that address the book's focus. Granted, understanding how OS X works is the best "first step" you can take towards knowing how to fix it when it doesn't work; in that light, after reading this book you should be a better troubleshooter than when you first picked it up. However, I wanted to concentrate on how to make your Mac work better, rather than how to get it working when it doesn't. If you're looking for a book on troubleshooting, the best one out there, in my opinion, is Mac OS X Disaster Relief by Ted Landau (although I'm biased, since I wrote part of it). It's also one of the best books on the market at explaining the intricate details of the OS. In addition, Apple's Knowledge Base has a helpful article on troubleshooting OS X at http://docs.info.apple.com/article.html?artnum=25392.
Second, as I mentioned earlier, this is also not an "absolute beginner" book. I've tried to make this book accessible and useful for many types of users, from beginners to power users, so I've tried to bridge the usual gap between "learning OS X" books and "tips and tricks for OS X" books. I touch on the basics the first time I mention each topic, and then move on to more advanced info. However, I assume that you've used a computer before, even if for just a little bit, and that you're familiar with the basics: turning it on, opening a folder or document, using menus in applications, dragging files around, etc. If you're not comfortable doing these types of things, I recommend spending a bit more time with your Mac before you dive in. If you want some recommendations for books to get you started, my favorites are Mastering Mac OS X by Todd Stauffer and Mac OS X: The Missing Manual by David Pogue. Both take you through the most basic of basics, but also delve into more advanced topics when you're ready.
Third, this isn't a hardware book; it's a book about the operating system and software that is Mac OS X. There may be occasions here and there where I point out certain hardware features that are (or are not) supported on various Mac models, but that's about it. I won't spend time talking about your Mac's hardware, how to install new hardware, and how to fix hardware problems.
Finally, this book isn't comprehensive—and I mean that in a good way! If you don't see a way to do something in this book, that doesn't mean it can't be done. Likewise, if you do see a way to do something, that doesn't necessarily mean it's the only way to do it. There are a lot of other ways to get more out of OS X, and that's the beauty of it. I've simply tried to put together what I feel is a great collection of information, tips, tweaks, and software gems that will help you get the most out of your Mac. If I'm lucky, the background and ideas you get from this book will inspire you to think of even better ways to accomplish the things you need to do. Websites like http://www.macosxhints.com/ are filled with new tips and information on a daily basis.