All Beej's guides are fantastic, but if you're interested in Network programming you could do way worse than follow this up with his guide to network concepts. https://beej.us/guide/bgnet0/ If you search Algolia for Beej you'll see his material has been on hacker news numerous times.
Anyone knows of a well-rounded guide that explains the structure of the Internet at large? I know everything listed in the one you linked, but I have a nebulous idea of how anything works after my packet exits my gateway. So, BGP, autonomous systems, peering agreements and other site-to-site routing protocols that keep the entire Internet infrastructure ticking along.
I thought "How the Internet Really Works" (https://nostarch.com/how-internet-really-works) was pretty good.