I'm writing https://github.com/controversy187/kerbalbook, which aims to be a hobbyist's guide to orbital mechanics, spaceflight, and maybe astrophysics. All this through the lens of a Kerbal Space Program walkthrough.
I'm building https://adeptli.org with the goal of making personalized learning available to anyone with an internet connection. It's pay-as-you-go, and everyone starts with free credits to check it out and not incur costs if they don't find it useful. I'm still very early in development and very open to feedback!