I'm working on a git(1) implementation written in Python. I'm following along the amazing James Coglan's book. Currently 23 chapters in, about to finish off the Part II of the book. Can't wait for the final, third part.

I love books like this one. Some other examples are "Crafting interpreters", "Writing a C compiler", "Building a debugger", and a couple other lower quality ones. The potential in this space for aspiring technical writers is enormous. Let me know if you know some other books that guide you through implementing complex systems software from scratch.