I've yet to break into this one (I've seen it recommended so many times on HN). I'm no network engineer, but networking is a lot of fun for me and I enjoy learning about how it all works. High Performance Browser Networking[0] was one of my favorite books on this topic. Very approachable, even to complete beginner. I suspect that Beej's guide is a bit more low level and I look forward to that!
I think HPBN and Beej's guide are perfect companions to each other. They've been the most useful resources on my journey to learn more about networking.
[0]https://hpbn.co/