The symfony and Doctrine book - a brief description of a few critical Doctrine subjects that symfony utilizes the most;
Symfony API - detailed API documentation for the Symfony framework codebase. The best way to check the parameters of a function or the nature of the returned value is there;
A Gentle Introduction to symfony - this book introduces you to symfony, showing you how to wield its many features to develop web applications faster and more efficiently, even if you only know a bit of PHP;
"Practical symfony" tutorial is a great way for a beginner to learn symfony, understand how it works, and also see best web development practices in action;
The More with symfony book - this book is a collection of tutorials about various and more advanced symfony topics;