This specific link has been on my to-do list for so-long now that I’ve decided to just share it without any further ado.
The people behind GoalKicker, for whatever reason, decided to compile nearly 100 books on different programming languages based on among others StackOverflow entries. Their open access library contains books on languages from Latex to Linux, from Java to JavaScript, from SQL to MySQL, and from C, to C++, C#, and objective-C.
Basically, they host it all. Have a look yourself: https://books.goalkicker.com/
paulvanderlaken.com 