The tool I use to render all the markdown files in this wiki.
In my opinion this is the future of book and publishing in general. Being able to put writing under Git and have such an awesome way to render what you write and have a builtin search ready to use, all for free is incredible.
Having a book you write be available on the internet, ready to be read in an instant is amazing. I believe all books and knowledge should be free with pay as much as you like as a thank you attached. The bonus of publishing your things on GitBook or LeanPub is that not only do you get the web rendering and you can just focus on the content and writing, but you also can export it to different formats like PDF or EPUB if you so wish. And of course if your book is open source as it should be, anyone can edit it and improve or fork it and make something else and extend it in a way they wish.
Similar to how I took Yoshua's idea with his original knowledge repo, I modified it, forked it and made my own. That's how books should be written or well, can be written. There is no right way of doing of things but I think Git and Web publishing as first class citizen is pretty damn awesome.
Cool things GitBook has
Plugins - Extend your GitBook with useful things.