Not a prep course, but for example Redis has a very good source code overview here https://github.com/antirez/redis#redis-internals

More remarkable is the fact that antirez updated the documentation in response to a post in Reddit. https://www.reddit.com/r/redis/comments/3re0aw/any_pointers_... Thank you antirez! :-)

Also check out libpng's source.

https://github.com/glennrp/libpng

Oh wow. That's beautifully done. Simple comments that explain clearly what the code is doing, pretty clear choice of variable names so little head-scratching going on.

The documentation of redis is really good for a large open source project. I am not a contributor, but still read the source code from time to time. Full credit to antirez for taking the time to make it easy to contribute to redis!

Shameless plug here but I have put together a free ebook detailing the inner workings of the python virtual machine @ https://leanpub.com/insidethepythonvirtualmachine

I agree - this basically gives you enough information to bootstrap your own learning about the CPython internals. I feel like all companies/projects should have a similar intro which gives new-joiners enough information that they can figure out most things themselves without too much pain, and without spoon-feeding too much.