If you have money you should consider supporting http://en.goteo.org/project/elenq-publishing
CC licensed technical books translated into multiple languages, they only need about 200 more euros to get a professional translator for the current set of books.
I may be a bit biased because @ekaitz_zarraga@mastodon.social is awesome, but whatever.
I am also a bit biased because I want to try and write similar things about the more accessible parts of my phd research that have direct applications in lots of stuff people don't realise. Like practical linear algebra which covers any graphics systems, movement in games, 3d representations of objects for games or art, also just about any communication system.
Also information theory and probability for when you want to count cards and figure out the least terrible way to bet on something.
you could use all of that for things like building distributed control systems for mesh networks and security stuff, or adaptive algorithms and machine learning, but that is probably a bit less accessible.