No.1732
Rather than buying books you could just read the RFCs that introduced the protocols. Most are very readable, they explain the why, the how, the history and so on.
The only thing you don't get from the RFCs themselves is a filter of what's considered important or not, but you can probably just start reading about the protocols and systems that you use (http, dns, stp, smtp), finding out which ones are widely used is usually easy enough since they're being mentioned all over the place.
You'll be studying from the authoritative source, I'd say you can't go wrong there.