https://github.com/vhf/free-programming ... g-books.md
The "Free Programming Books" collection on Github is an aggregation of various links from around the web covering just about everything out there including some topics that probably don't fit well with traditional programming languages. All of the resources point to free ebooks, sites, or other sources that can help you on your journey to becoming a programmer (or a better programmer). Of course, these free ebooks are no substitute for authoritative textbooks, but more often than not a simple tutorial can push you well along the way to establishing what you don't know, what you need to know more about, and point you to other resources that actually do help. I've been using some of these with learning Golang, for instance, and they're quite good (though as with any new language, there's a relative dearth of usable resources).
Before you dismiss any of this outright, note that some of the links here cover topics that you might not expect. R, SQL, and even OS-specific resources (Windows 8, iOS, Android, or Unix/Linux shells and assorted tools) are linked, so in many cases, this might be a good place to start if you're in need of additional reference material. In particular, for topics (like Python Flask) that have few (if any) published texts, what is listed here might be your only hope outside documentation. Or in the case of Go, the language specification.
If you're curious about learning another OS, this list even contains a number of free books on Linux (and Ubuntu, in particular). So if you--or someone you know--happens to be looking for assistance, browse the directory and see what you can find. It's growing every week, and I suspect this might just evolve into the authoritative "free" resource collection.