[_bernd]

> 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/

[_bernd]

The link to https://book.systemsapproach.org/internetworking/routing.htm... is in the first paragraph.