As the other reviews have stated, this is a very good book. I had been looking for a book like this for a while, so I first picked this up in a beta version from the Pragmatic Programmers website.
The two first section give a wonderful introduction to TDD in C. By the end of the second section, Grenning has covered the reasons for doing TDD, looked at available tools, and introduced various methods (spies, test doubles, mocks) for breaking module dependencies during testing. Lots of code examples are included throughout. These two sections were by far the most useful to me. Having been a programmer for a number of years without doing TDD, I needed some convincing, so the "Yeah, but..." chapter was spot on.
The third section (Design and Continuous Improvement) feels a little bit more unfocused. It covers three rather large topics (SOLID design, refactoring, and working with legacy code) that all deserve (and have) whole books dedicated to them. It may be intended as further examples of how to apply TDD, and it does do a fine job of that.
In short, I think this book serves as a very good introduction to the topic. That does not mean, however, that it answered all my questions about TDD. Most of these question revolve around how these techniques scale up to larger projects and teams.
* In Chapter 10 it is stated that "Mocks enforce a strict ordering of interactions, which can lead to fragile tests ...". I would have loved to read some thoughts on when this is likely to occur, possible solutions, etc.
* The LED driver example is a good example, but it isn't immediately obvious how this approach would scale to larger hardware blocks (say, a co-processor).
Also, performance concerns are mentioned a few times, but may have had deserved a little more space. For example, in Chapter 3 it is stated that abstract data types are hidden (only forward declared in the header) from the caller. In its naïve form, this does not allow for inlined function calls, which can still be a performance problem on some platforms. A discussion on how to deal with issues like this would have been useful.