The third edition of the book Physically Based Rendering: From Theory to Implementation is now available for free online at http://www.pbr-book.org. The book by Matt Pharr, Wenzel Jakob, and Greg Humphreys describes the mathematical theory behind a modern photorealistic rendering system as well as its practical implementation. 

In 2014 the authors earned an Academy Award for Scientific and Technical Achievement from the Academy of Motion Picture Arts and Sciences based on the knowledge shared in the first and second editions, and the Academy called the book a "widely adopted practical roadmap for most physically based shading and lighting systems used in film production."

Learn more and browse the contents at http://www.pbr-book.org/.