The lectures are excellent for reviewing the book. Since the lectures cover almost 90% of the same content as the book, you can learn enough from the lectures alone. However, I recommend purchasing the book as if you are paying a tuition fee, reading it first, and then watching the corresponding lecture chapters to review. To be honest, other layers in networking are mostly for certification exams or general knowledge, whereas HTTP is what you actually need to know in detail. I am very satisfied because this course explains all those practical parts. This instructor's lectures are of much higher quality and better for learning than the networking courses taught at most universities. Just because it's a four-year university in Seoul doesn't mean they teach something incredibly grand. This course is simply easier to understand. It is much more substantial than what you would learn in a full semester at university. I think the only difference from university is the amount of knowledge forced into your head due to assignments and exams. If you want to dive even deeper, I suggest purchasing a "Top-Down Networking" textbook, which is commonly used as a major requirement, and studying further. These lectures alone are more than enough to gain all the fundamental knowledge.