What are the most important books you've read about open source software?

I've been collecting (as well as editing) books on this topic for a long time. Here is a list of the ones I feel are truly the most relevant and informative. They're not about individual technologies, but about aspects of the whole movement. A lot of these books, appropriately, have free online versions. If this list seems heavy on O'Reilly books, just consider all the relevant O'Reilly books I did not mention.

• The Cathedral & the Bazaar, by Eric S. Raymond, O'Reilly Media, ISBN 978-0-596-00108-7. The first serious attempt to understand socially and economically how free software works. Eric has contributed to free software for a long time, and he also has training in anthropology, which helps. His social analyses also undergird a lot of what he wrote for the Hacker's Dictionary. Many individual points in The Catheral have been challenged, but it remains the central statement of the movement.
  
  
• Code: And Other Laws of Cyberspace, Version 2.0, by Lawrence Lessig, Basic Books, ISBN 978-0465039142. This classic, the first (I think) of many books by Lessig on information freedom, explains where software fits into law and regulation. It's the basis of the modern movement toward opening all sorts of systems, and of Lessig's Creative Commons.
  
  
• CODE: collaborative ownership and the digital economy, edited by Rishab Aiyer Ghosh, MIT Press, ISBN 0-262-07260-2. Oh-oh, poor coordination--two books on a similar topic with the same name. This is an academic work that's more about anthropology and law than computing, but interesting to see the different ways sharing has been viewed through history.
  
  
• Intellectual Property and Open Source A Practical Guide to Protecting Code, by Van Lindberg, O'Reilly Media, ISBN 978-0-596-51796-0. Discusses all the important topics that programmers think they know and really don't. Preserving your rights to your work, understanding patents, trademarks, and copyright, etc.
  
  
• Open Source for the Enterprise: Managing Risks, Reaping Rewards, by Dan Woods and Gautam Guliani, O'Reilly Media, ISBN 978-0-596-10119-0. A book for planners and managers, talking about such things as how to determine whether software is stable enough for use, and how to train staff to deal with free software communities.
  
  
• Producing Open Source Software: How to Run a Successful Free Software Project, by Karl Fogel, O'Reilly Media. ISBN 978-0-596-00759-1: This book is truly practical, and covers all sorts of angles from tools to community to funding.
  
  
• Wealth of Networks, by Yochai Benkler. Yale University Press, ISBN 9780300110562. To tell the truth, I can't determine how original this book is, because I felt like I'd heard it all already, but maybe that's just because everybody else I've reading have been repeating Benkler. Oh well, it's a fixture of the open source movement now, so you'll find the key themes here. (For a shorter essay on the theme, try Coase's Penguin.)

What a fantastic list! Very well thought out and informative. Thanks for posting this, Andy. And here I was cleaning up some comment spam. This totally deserves a wider view.

I had to pass a test the last month and my mentor suggested to read the book written by Van Lindberg "Intellectual Property and Open Source A Practical Guide to Protecting Code", when I put my hand on it I could not let it go because it contained so much useful information. A friend of mine suggested to read the "Wealth of Networks" book, I heard the first time about it and decided it is not so important, but if this book is included in the list you provided it means it is very good so I will surely read it in as soon as I will find it.