I remember looking at the Ruby source code about a year ago, so it may have drastically improved since then, but I remember it being a bit of a mess. But it is quite fun to step through the code in gdb and you can learn quite a lot by hacking on it or implementing a DLL extension.

If trolling through language implementations is your kind of thing, I'd suggest having a look at Lua or Python as well (I never got Perl to configure, let alone compile) - then you can compare and contrast with the way things are done in Ruby.