I'm glad you start from a 'what you'll need' point of view. This is often missed in most books where they just start with a code 1+1 example.

If you are aiming for total beginners - as in, new to programming and not just new to Ruby/Rails then you might have to re-word your About this book section.

You mention TDD, Git, 'default Ruby stack' within the first 8 lines of the book. While these are important points regarding the book - they only make sense to programmers of some kind.

I think if you give away a sample chapter of your book then this will be the first thing they read. You've got to convince the new folks you'll look after them & that they will actually get as far as building their first app. You may want to consider adding a book subtitle. I presume the readers you want to attract might not even know what Rails is! Maybe something like - Anyone Can Ride Rails - A fresh programming guide for enthusiasic beginners.

I like the informal tone as it's reassuring. I also second the what is Ruby/Rails & why should I use them, what can I build with it etc?