Depends on how you prefer to learn, but here are a few suggestions.

I've heard good things about the Computer Networks book by Tanenbaum and Wetherall, but I haven't read it myself. Very broad and comprehensive. The most hardcore way would be to make reading RFCs your hobby. It can be tough to get through, but if you regularly take half an hour to do it, you'll learn so much. I've recently started a course at https://classes.pracnet.net/, which is good too.