Also see All About Peer-To-Peer Architecture in Webopedia's "Did You Know...?" section.

Peer-to-Peer Communications Inc.
Features technical titles on programming, operating systems, hardware, software, and the Internet. Includes non-fiction books about the industry.

Gnutella Web site
Originally developed by Nullsoft, Gnutella quickly disseminated within the hacker community.

What Network Do You Need?
An overview of the different types of networks possibilities, from peer-to-pear to client/server, and what kind of products you need to run them.