Game development and design are some of the most extensive works of art around. This is due to the large amount of expertise that is required to bring a game to life. You can get an idea of this by just looking at any credits in a game. They are extensive and contain a lot of names of people who have dedicated a lot of their time to the game in various roles.
Like most things in life, game development can be learned not only through practice, but iteration as well. And even when you master one of the many branches in game development, there is still something new to learn.
Regardless of your level of knowledge of Unity, I strongly suggest that you follow each step in this book, even if you think you know the topic. You just never know, there is always something new to learn!