“An Introduction to Ray Tracing” is now free for download (2019)
realtimerendering.com
realtimerendering.com
(edit: I realize it's in the article, but it deserves highlighting here again. This is a really good book.)
Indeed! PBR is by far the best technical books I've read.
PBR derives the math and goes on to show how to implement it, all in a very understandable way, which at least in my experience is quite rare.
A lot of math is non-trivial to implement in a correct and well-behaved manner. Even just solving quadratic equations is not straight forward[1].
[1]: https://www.johndcook.com/blog/2010/06/02/whats-so-hard-abou...
Is the algorithm in [1] significantly more precise for inputs that don't result in overflow?
Edit: I made a small program to check a few values. It is seems like the algorithm in [1] is better in terms on precision, but only by a small margin (on the small set of values I checked).