If you want to learn WiX, but are not sure whether you need to spend money on this book instead of relying solely on free resources available online, do yourself a favor: buy the book. It will explain the basic concepts of both the Windows Installer technology and WiX development better than other tutorials, wikis, and help guides you may find (at least based on the resources I found by the time of writing this review). Not only does it describe what you need to do to accomplish certain tasks, but it also explains why you need to do this and that. It is definitely not a comprehensive guide, but for its size, it covers a lot of topics.
I have some experience with MSI, InstallShield, and related installer technologies, but my knowledge of MSI internals is very limited (before switching to WiX, I mostly used Visual Studio Installer projects for building installers). I started learning WiX the hard way: reading user guide, tutorials, wiki; and in a couple of weeks I was able to write several installers for internal projects. Nevertheless, I still had very vague idea about the implementation (i.e. what does this property mean, why I need to include this attribute, what do these elements do), so I bought the book (well, my department did pay for it, but if not, I would've paid myself). The book cleared a lot of things for me.
The book covers the fundamentals extremely well. What is a product, package, media? How do they relate? How do you use features, components, files? The book answers these and other questions in more detail than you would find elsewhere. I found the description of major upgrades (e.g. the matrix explaining the effects of scheduling removal of older version at different execution events) particularly helpful. Chapters covering user interfaces (using default wizards, modifying existing dialogs, customizing wizards) are very informational. So is the discussion of localization. Examples illustrate the topics nicely. The book offers sensible recommendations (e.g. avoid per-user setup packages).
A couple of things I found somewhat confusing include description of properties, variables, etc, when it's not clear whether these get resolved during build time or deployment time and issues pertaining to 64-bit vs 32-bit installers (e.g. how system folders get resolved on various combinations of platforms and installers: 32-bit MSI/64-bit OS, etc). I wish the index were more comprehensive; most of the time when I needed to find something, I just flipped the pages.
I agree with points made by other three reviewers, so I won't repeat them. In short, if you're planning to use WiX to write installers, get this book; you will find it quite helpful.