Here is a collection of articles, tutorials, and whatnot that help with understanding of CPython internals and source code.

###Python Documentation

• Python Developer's Guide

• Extending and Embedding the Python Interpreter

• Python/C API Reference Manual

###Blog Post Series

• Eli Bendersky's Python Internals series

  • Adding a New Statement to Python

• Yaniv Aknin's Python Innards series

• Allison Kaptur's Python Internals Series

• My Python Internals Series!

###Other Various Blog Posts and Papers

• Python Compiler Internals, by Thomas Lee

• How Fast Can We Make Interpreted Python?, by Russel Power and Alex Rubinsteyn

• Python Attributes and Methods

• Understanding Python by breaking it