I just bought a copy of Physically Based Rendering, I've been meaning to get one for ages as it's often recommended and I thought it might be useful given my recent interest in global illumination. I'm also hoping to get a more formal background in rendering rather than the hacktastic world of real time.
The subjects covered are broad and it's very readable. It's my first exposure to literate programming, where essentially the book describes and contains the full implementation of a program. In fact the source code for their renderer is generated (tangled) from the definition of the book before compilation.
The only problem is the amount it weighs, I like to read on the tube but 1000 page hard backs aren't exactly light reading.