Albeit being an old book (2003) it is still a milestone in the victorious path of Test Driven Development. It basically grabs your hand and walks you away from the shadowy lands of ugly, unmaintainable code to the land of the beauty and efficiency of test driven development. It gives you compelling arguments and extremely clear examples how development driven with tests guides you towards the simplest yet elegant and capable design.
A must buy for people needing a thorough walk through of the quite rigorous TDD development cycle also known as Red-Green-Refactor and a priceless reference for seasoned developers as well.