I don't know whether this strictly meets your criteria of a "book". I'll let you decide.

This is one of my favourite references for latex, since it actually covers a lot of topics, and also references a lot of packages (not only core latex)

Under the Language Agnostic section, I've added a heading for standards documents, and started by adding the POSIX standard, which links to some instructions I've posted on how to access it without paying. I actually have a number of these, if you think it's the type of thing that would be useful on this list?

Added the list from https://code.google.com/p/google-styleguide/, which includes C++, Objective C, Java, Python, HTML/CSS, and Common Lisp. Also added is Ian McDonald's Ruby guide, which was used internally on an unofficial basis but is also thorough.

a free online book about AutoHotkey. AHK is free, open-source scripting
language for Windows - more at www.ahkscript.org and www.autohotkey.com

The removed line corresponds with the Haskell Wikibook link which is already present. (The wikibook link is usually more updated than the removed pdf version. Also you can download pdf version of this book from the wikibook link.)