If you're interested in the mathematics behind PBR renderers like Cycles, then this book might be for you. The authors Matt Pharr, Wenzel Jakob, and Greg Humphreys dive deep into all aspects of modern renderers. They start with basic concepts like coordinate systems and vector math and work their way through everything you need to know to build your own renderer.
This book presents a selection of modern rendering algorithms through the documented source code for a complete rendering system. Nearly all of the images in this book, including the one on the front cover, were rendered by this software. All of the algorithms that came together to generate these images are described in these pages.
'Physically Based Rendering: From Theory To Implementation' is released under a Attribution-NonCommercial-NoDerivs 4.0 International license.