Node.js programming means you have to choose which modules to use. Beyond the basic "Hello world" web server you start making decisions about which modules to incorporate to your project right away.
So it's hard to learn Node.js when the author you're following chooses some technology you don't quite like or if the author chooses a path of subjects you are not comfortable with.
This book intent is to allow you, the reader, to choose what to learn or even compare different modules and technologies. So beyond the basic chapters that will teach you things you'll need to know anyway, it's up to you what to choose next once you end up a subject.
Maybe you want to dive into databases right away, or maybe your project doesn't need a database at all and you want to know about handling sessions right away.
Well, the driving idea of this books is that you can choose what to learn next. And as in the popular book series "Choose your own adventure" (http://www.cyoa.com/), you'll make your own path.
Since this is a huge work, and I'm just starting with it, you're more than welcome to contribute as long as you agree with the LICENSE, so please go check it out and then fork this and make me a pull request ;)