What about k?

This is the beginnings of an introduction to the k language, primarily in its open source form.

It's very much a work in progress, and not completed. It's written as a jupyter-book, with the kernel for ngn/k available here

You can view it in its current form on https://xpqz.github.io/kbook

I've written a few other things you may be interested in, too:

• Learning APL
• APL Cultivations