Peter Shirley's Raytracing in a Weekend minibooks are another good introduction that goes beyond rendering spheres, and they are now available for free:
Huge endorsement for this one - I’ve been working my way through it and I’ve found that this has helped me level up in my language skills in a way that no other method has so far.
How much of this has actually been written? Can't seem to find any information on current progress beyond the releases section (which notes 4 additional chapters in the last couple months, but little about existing chapters), and googling doesn't immediately turn up any kind progress-blog. I'm not assuming in-order writing, since I've seen other `beta` e-books not follow it (and it often makes sense not to)
So far the pace seems to be about two chapters a month, with the update today bringing the total up to 14. According to the TOC, that only leaves three more to go, plus revision and editing. It is being released in-order - following the author's twitter and screenshots posted there over the past two years, it looks like the content is complete.
I bought the book in beta, chapters 1 - 14 are complete, the last 3 chapters are in progress. More than enough material to keep you busy until the author finishes the last 3 chapters.
TDD is a great approach for a raytracer. They can be absolute nightmares to debug when a minor error in some calculation causes subtle artifacts in your image.
https://pragprog.com/book/jbtracer/the-ray-tracer-challenge