http

Web Application Architecture: Principles, Protocols and Practices
$45.15
This book has a lot good information but I was disappointed that there wasn't more information about Java EE 5, EJB 3.0 and the Java Persistence API. The book only barley mentions them.

HTTP Programming Recipes for C# Bots
$39.99
I've had this book for a few months and it is the best published bot book.
It tells you need to know about the http protocol to extract data from a site which use javascript and ajax! The parsing libraries are very help, though some tweaking is always necessary given the magnitude of sites.
If you need to write a bot, but don't know how, this book it for you.

HTTP: The Definitive Guide
$44.95
I bought this book after reading some preview chapters. The book is actually a bit 'old'. It was first published a few years ago, but HTTP hasn't changed that much from that time. Something I like about this book is that it gives you links to more info at the end of each chapter.

The book is really easy to read, and it has diagrams to help you understand the processes that http and it's related technologies follow. It talks about optimization, proxy servers, security, gateways, tunnels, web robots, secure http and much more. It even has a script to write a small http app. If your goal is to fully understand the http protocol, this is a great book to start.

HTTP Essentials: Protocols for Secure, Scaleable Web Sites
$34.99
You should not try reading this book from start to finish - that may be a little too much. As a reference, it is the best HTTP book I was able to find. Clear and concise information compiled from multiple sources.