Data and Computer Communications – by William Stallings – not my favorite book, however this is a fully complete book on all aspects of network protocols. We’re using it in my graduate course “Advanced Network Protocols and Standards”.
Because the Stallings book is not the easiest to get through, I supplement my reading with “Computer Networks and Internets (with Internet Applications)” By Douglas E. Comer. I find it a little more readable. I had the 3rd edition around my house from my undergraduate days, so I’ve been using that.
As a student, I’ve found that between both of these sites, I’m able to get most of my textbooks for free:
https://libribook.com and https://1lib.us/
