What Every Web Developer Should Know About HTTP has a long title, but it’s a pretty short read. I didn’t time my reading, but my Kindle tells me that 30 minutes remain in the book if I open to the first page.
I bought this book because I’ve considered writing something similar concerning other topics like DNS or SSL, and I can report that it’s a decent tour through HTTP.
If you’ve never made a HTTP request by running telnet against a web server, or never reviewed requests and responses in Wireshark or a developer console, this book is for you. If you have a junior developer on the team that’s just learning about web programming, send them a copy of this to get them started. Examples are in C#, so stay away if you hate C# and you’re too shallow of a programmer to look past that choice (and consider yourself judged for that shallowness).