I don't like that this book is posted. Sure, it shows you an actual renderer. But it doesn't show you how to incrementally build said renderer. That is probably what a lot of people are looking for. Peter Shirley has written three short books about raytracing that I would recommend as an alternative.
I've read Realistic Ray Tracing by Shirley, before reading the first edition of PBRT. RRT definitely has a more "incremental" approach, and is a lot more approachable than PBRT.