This is my favorite text on electronics, open or otherwise: https://www.ibiblio.org/kuphaldt/electricCircuits/

The explanation of impedance (volume II, chapter 14) was the first one that made intuitive sense to me, after struggling with the concept for years. The whole thing is beautiful, but that chapter especially.

The thing that disappoints me about Openstax is that, while they're free/libre, they don't seem to be open source anymore. They used to provide access to the raw XML (CNXML) used to generate the books, but I can't find that anywhere.

It would be nice if there was something like OpenStax, maybe even a fork of it, that could be held in a Git repo and anyone could make pull requests.

I highly recommend Interactive Linear Algebra for learning undergrad-level linear algebra mostly in the R^N. It has a PDF version available, but the real magic is in the interactive embedded 2D and 3D visualizations.

There's a couple of variations available - it was written by some professors at Georgia Tech, one of which moved to Duke and teaches off of it there - but the UBC version is my favorite, as it makes some changes in the lesson order to teach vectors before matrices.

https://www.math.ubc.ca/~tbjw/ila

Openstax is the best source of extremely high-quality alt descriptions I've ever found.

If you need inspiration for good alt descriptions of STEM / computer science / software engineering related images, just look at how Openstax does it. I'm a screen reader user myself and I'm very impressed.