Kent Beck is one of the most known proponents and "spreaders" of eXtreme Programming, and is the author of the excellent "Extreme Programming explained" book. In "TDD by example", Beck tries to expand on the topic of test-driven-development (in short TDD), providing some examples and "testing patterns".
Unfortunately he failed in this task. "TDD by example" is a blunt, boring book which adds nothing new over the "Extreme Programming explained" primer. The examples are trivial, over-chewed and don't teach almost anything useful. The philosophy he presents is way "over-extreme", and I seriously doubt that anyone programs like that. What us is really in an example that "tests" the function add(a, b) which performs... that's right - addition of a to b. Beck "test drives" by writing the test "assertEquals(add(2, 2), 4)" and "just to see the green bar" implements add(a, b) as "return 4". Come on !! What does this teach ?
The last part of the book, concentrating on "design and test patterns" promised to be interesting but proved to be just a dry recycling of some points from Fowler's "Refactoring" book. The only thing I did learn from it was some interesting usage of Mock objects. At least it wasn't completely useless :-)
All in all, if you're even a little familiar with XP, this book is a waste of your time. If you aren't familiar with this method of development, by all means read Beck's fine "Extreme Programming explained".