Free for All: Full text of Peter Wayner's book, subtitled "How Linux and the Free Software Movement Undercut the High-Tech Titans". [PDF]
Open Sources: Voices from the Open Source Revolution: The full text of this O'Reilly book has been made freely available online.
Producing Open Source Software - How to Run a Successful Free Software Project: Book by Karl Fogel. Full text freely available as HTML, PDF, and XML.
The Linux Cookbook: All-encompassing book on using open source software, which itself is open source and available online.