I hesitate to suggest "just one book" but I think that for Ruby on Rails, this will get you pretty far:
3. Work your way through "Agile Development with Rails" 4th edition (beta)
http://pragprog.com/titles/rails4/agile-web-development-with-rails
Mind you... if you don't know ruby, you risk running into confusion between what bits are ruby and what bits are rails, and a lot of things in rails will seem very mysterious... like magic. That said -- there are enormous amounts of tutorials and help available online, so it's certainly possible to just go ahead and start.
Also, Chris Pine's book actually started out as a tutorial online, and it is still available at his website, here:
http://pine.fm/LearnToProgram/
So my revised suggestion is:
1. rubykoans.com (free)
2. Chris Pine's online version of his tutorial (free)
3. Agile Development with Rails, 4th edition (worth every penny)
4. Have fun, and read LOTS of code on github!