The CPython source is really a bit of a joy to look through. The lowest level stuff is really tough, but generally it's all relatively straightforward. Though

The CPython source is really a bit of a joy to look through. The lowest level stuff is really tough, but generally it's all relatively straightforward. Though with the specializing compiler work, some stuff is... less obvious. But I still generally find it straightforward when I want to know some detail about how the language works.