I have a question about this book. How are you supposed to follow along with it? Peter Shirley's book encourages the reader to program the renderer along with the explanations. It doesn't seem like that is possible with PBRT, but I might be misunderstanding something.
PBRT is big because it is written using literate programming and therefore includes (almost[1]) the entire source code for the ray tracer in print, which arguably makes it easier to follow than other books on the topic.
The PBRT ray tracer is also a production quality ray tracer and uses many advanced techniques you won't find hobby ray tracers, such as differential geometry, which naturally increases it's size.
There are exercises at the end of each chapter. Most exercises ask you to implement something new in pbrt, often something described in a (SIGGRAPH) paper. Pbrt is very modular and it's quite easy to add a new acceleration structure, a new shape, a new light, etc. I haven't done these exercises myself though, I mostly use the book as a reference while implementing my own renderer.
pbrt is a raytracer code + a book to explain the code and the maths involved. You don't really need the code to read and understand the book. It's an amazing idea and his award is truly deserved. The point is not to learn you how to _write_ a raytracer but to explain how a production raytracer _works_.
