HTTP Illustrated
In TCP/IP Illustrated, Richard Stevens describe the TCP/IP protocol stack by generating interesting situations and looking at what happens on the wire with tcpdump. Don't look for pretty illustrations, "illustrated" here means "by examples".
This is an excellent set of books, a bit dated but the technique lives on. Learning this stuff by just reading the RFCs must be painful. For some reason the RFC editors want the official documents to be plain ASCII with manual page breaks. For the nostalgic it must feel good to read document in 2006 that look exactly like a bunch of typewritten pages from 1966. The RFC editor even recommends using troff to "typeset" the document... Let's hope that they hear about Docbook one of these days. OK Docbook is not that nice, we can blame XML's verbosity for most of the problems and the rest of the blame goes to the extremely fragile tool chain that is considered "standard" on most GNU/Linux distributions. But at least with Docbook you can hack your own tool chain and you'll end up with stuff that don't look like crap.