I'd not come across this before; at first glance, it looks like a good resource for beginners. The lack of clear authorship makes me a bit uncomfortable, though, since it's optionally available as a paid purchase. FWIW, the GetContented username has only been around on reddit for 3 months. I wouldn't want to end up in a situation where we're recommending a paid product that might very well disappear before the book is finished, even if we're only linking to the free version.