Deja Vu.
I read this article a few days ago I'm sure, word-for-word, but it wasn't on this site in OP? It stood out because when it mentioned textbooks and said "including ours" I looked at the site and thought to myself "they do textbooks?".
This submission was made three days ago and bumped two hours ago: https://news.ycombinator.com/submitted?id=drbruced
> and thought to myself "they do textbooks?".
Indeed: https://systemsapproach.org/books-html/
If you are cheap on money, but you do have time, and like to get into networking, I can only highly recommend https://book.systemsapproach.org/