80/20 rule. We only talk about that 20% of skills that deliver 80% of the value. A reader will have much less stuff to learn but much more to gain.
One skill at a time. Nobody can learn many different skills at the same time. Therefore, we talk about only one skill at a time and do not move on until the readers master it.
Learn by doing. Nobody can learn how to swim by just reading a book. Similarly, nobody can learn how to develop software by just reading a book. Therefore, we have packed a lot of exercises into the book for the readers to practice.
Feedback. When learning to swim, it's best to have a coach to point out the problems in our postures. Similarly, someone learning to program would benefit much from feedbacks. Therefore, we provide sample solutions to the exercises so that the readers compare their solutions to ours.
Reinforcement. After learning to swim, without frequent practice one will simply forget how to do it. Therefore, the exercises in the later chapters frequently test the readers on the topics taught in the previous chapters, so that they can really reinforce their newly learned skills.