A few observations. One, you haven't written that many pages, in fact you've written very, very few pages save for the preface and the like. This isn't an issue on its own, except theres no Table of Contents so it's hard for me to see where you are going to take it from here, or in other words what your vision is. The obvious result of this is that it's pretty hard to provide feedback. The flip side is of course, that you've demonstrated enough desire and initiative within yourself to endeavor to start the book, so unless you have a serious reason to stop, maybe forge ahead and try to finish what you started.

One other thing that I just want to put in your head, I'm not sure if I'm even going so far as to suggest it, is the possibility of flexing scope to write perhaps a 50 page e-book, not dissimilar to the Sacha Greif ebook (http://sachagreif.com/ebook/) but for getting from zero to the next book on Rails for absolute beginners. The benefit of this would be that you could probably spend about 20 hours writing it (I'm pulling this number out of my ass, so I could be waaay off about this time estimate) and therefore you've risked a much shorter amount of time, which I'm assuming is your main concern.