Whilst not realtime rendering - Physically Based Rendering (Pharr, Jakob & Humphreys) http://www.pbrt.org/ http://www.pbrt.org/ is great for understanding what

Whilst not realtime rendering - Physically Based Rendering (Pharr, Jakob & Humphreys) http://www.pbrt.org/ http://www.pbrt.org/ is great for understanding what people are trying to achieve, without being distracted by very detailed optimisation etc.