Written material has been one of the most widespread methods to convey knowledge during the last millennia.
It started on a variety of objects, but we mainly benefit from it via printed books.
I myself still have a special taste in physical books: you can use them everywhere it's bright enough, can add personal notes on it with those useful tools called "pens".
Nowadays, information has increasingly taken a digital quality. Nevertheless, we still tremendously rely on digital books.
It seems that this questions:
Books to learn Ray Tracing has been put on hold because, if I judge from one of the comments, it involves book recommendation.
I find it concerning.
Why cut ourselves from a major source of information such as books or papers in our answers?
Why not include the vast space of the computer graphics related books in our exploration of this huge but exciting field?
I suggest accepting questions that are specifically oriented toward technical/scientific literature about our field of interest.
As once said a wise man (I think):
"Teach a man how to write a class, he'll make you an intersection module. Direct him to the relevant literature, he'll make real-time ray tracers all his life."