In this book, we will build the app from inside out. We will start with the model and then build the controllers and networking. At the end of the book, we will put everything together.
Usually, you would rather build apps feature-by-feature when doing TDD. But, by separating on the basis of layers instead of features, it is easier to follow and keep an overview of what is happening. When you later need to refresh your memory, the relevant information you need is easier to find.