I'm more or less finished The Ray Tracer Challenge, and it's brought me a ton of really fun hacking hours, and was a great introduction to ray tracing.
I tried to continue on with the Physically Based Rendering book (http://www.pbr-book.org/), but it's proving to be too advanced for me right now. I've been able to do some stuff in it, like some of the monte carlo path tracing, and some amount of physically based shading (lambertian reflection, some amount of microfacet reflection). It's a really big jump in difficulty.
Khan academy has also been a great resource for helping to bolster my math skills, which has made at least some of the math a bit more intuitive.
I thought I'd reach out for any good intermediate level resources, both on the math and 3d rendering front.
For myself, I only need to get through Chapter 10 and then the bonus chapter on image mapping. I've been working on an app for 2 years and really needed some ray tracing and I don't want to delay the app any more than it already has been by going through the entire book. I would love to . . . maybe I'll devote a small amount of time each week to it, I don't know.