Consider checking out https://book.systemsapproach.org/index.html as a free alternative as well.
The authors develop the book, as well as a few micro-books on topics like SDN, 5G, TCP congestion control, on github: https://github.com/SystemsApproach
They also have a blog with regular updates: https://www.systemsapproach.org/blog
And if you're curious about the motivation behind the project, they have a whitepaper: Open Source Networking Education: A Systems Approach - https://www.systemsapproach.org/uploads/4/7/7/4/47748257/sig...