Check out Pat Shaughnessy's Ruby Under a Microscope .
It gives a nice overview of the internals of MRI. It doesn't cover a who lot of the C code, but references plenty of it and where it can be found in the source code of MRI. Grab the book, the source for MRI, and do some digging.
Alas, I left before I could really get into it, and never had the peer pressure to understand a lot of core CS way over my head.