Prentice Hall PTR is proud to publish the Bruce Perens' Open Source Series with Bruce Perens, Series Editor. This Series focuses on Linux and Open Source technologies, including new and emerging technologies. It targets professional software developers, system and network administrators, and power users.
The Bruce Perens' Open Source Series is designed to give a voice to up-and-coming Open Source authors. Each book in the Series is published under the Open Publication License, an Open Source compatible book license. Electronic versions will be made available at no cost several months after each book's publication.
The books cover many useful topics, such as PHP 5, Linux programming and administration, Java, wxWidgets, Subversion version control, AJAX, etc. All of these books have free PDF versions available on this page.
Check them out; the price certainly cannot be beat.
Edit: A correction: some of the books (roughly 20%) have only a sample chapter available, not the full text!