[Question] Best open-source textbooks (goal: make them collaborative)?

I’m looking for online open-source /​ generously licensed textbooks, papers or tutorials.

Think of stuff like: http://​​neuralnetworksanddeeplearning.com/​​

Why? I’m currently running https://​​chimu.sh—a collaborative learning platform. Quick explanation: Chimu combines an e-reader with a Stack Overflow-like Q&A forum. As people read, they can view others’ questions and ask their own. Demo here (desktop works best).

I need to seed the site with initial content, and I figured LW would be a great place to ask.

With this in mind, what are good online tutorials /​ textbooks that people here have learned from? Is there any book or paper that you wish that you could discuss with your friends?