I would like to learn a couple of languages (polish and Japanese - I already learned Spanish to a decent degree) but it seems like every solution is closed source.
I would use books but listening (and speaking) are very important and books won't help with those.
The US Foreign Services Institute releases their learning materials, so if you're okay with a lack of structure I hear they can be very useful, including both reading and listening.
It can be accessed on several different sites. Here's one that came up, but you can find a few more with some searching: https://www.livelingua.com/fsi/