This is a comprehensive sub-section of the site of the Free Software Foundation and the GNU project; it covers its philosophy (as put forth by Richard Stallman and others) in detail. Several articles from there were references in the site, and there are many others of interest.
Contains many interesting articles and writing about Open Source, from one of its most prominent figures, including the immortal "The Cathedral and the Bazaar Series".
The Cathedral and the Bazaar - a document explaining the "Bazaar" way of managing a project.
Homesteading the Noosphere - an analysis of the "ownership" customs of the open-source developers community and their culture in general.
The Magic Cauldron - an analysis of the Economics of open-source.
How To Become a Hacker - a how-to document explaining how to become a "hacker", an expert and enthusiastic computer developer.
Software Release Practice HOWTO - explains good conventions in the management and release of an open source software package.
Joel Spolsky is a veteran software engineer that conveys his sometimes unusual views on software management and design in his site. Please don't take them too seriously, because he obviously doesn't either. Not all of his articles are relevant to the topic hand and here is a selected few which are.
Strategy Letter V - explains why big companies support open source and what's in it for them.
Five Worlds - explains the differences between the different "worlds" of software development and the rules that apply to each.
A free software consultant and analyst which has many interesting pieces of research on his site.
The Mythical Man-Month : Essays on Software Engineering by Fredrick P. Brooks, Jr. - one of the first books on software management by the team leader of the OS/360 IBM Mainframe operating system. Despite the fact that it was originally written in 1975 , it still contains many useful insights on software engineering.
The Cathedral and the Bazaar aims to explain how the problems raised by this book can be easily resolved.
Open Sources: Voices from the Open Source Revolution
This book is available online and gives the opinions of many of the most prominent leaders of the Free Software and Open Source world.