Recommended Reading List for PHP
Steve writes "IBM developerWorks has put together a PHP recommended reading list. It provides resources for developers and admins adopting PHP and tackling advanced topics such as building extensions and writing secure code. There's also a list of books and blogs for keeping up with changes to the language itself."
Design Patterns: Elements of Reusable Object-Oriented Software
Data Structures and Their Algorithms
Operating System Concepts
Joe Celko's Data and Databases: Concepts in Practice
How to Design Programs: An Introduction to Computing and Programming
Programming Ruby (2nd. Ed.)
Agile Web Development with Rails
+ some book on data communications and networking