Excellent Rails tutorial. It's frighteningly thorough, discussing Ruby, ERb, RSpec, Test-Driven Development, ActiveRecord, SQL, JavaScript, HTML, CSS, web design, REST, and of course, Rails. In only one book, you're walked through a mostly complete Twitter-like web application. The amazing thing is how fairly easy it all is to follow, even if you're not very acquainted with them.
The author doesn't miss a single detail, and all the explanations are very straightforward. He's obviously knows his stuff, not only about Ruby on Rails. The guy is a Harvard graduate, and an award-winning theoretical and computational physics professor at Caltech. And here is writing a basic Rails tutorial, so it's obviously a passion.
Although this book is impressively detailed for a tutorial, it is nonetheless only a tutorial, so don't expect to gain an in-depth understanding of any of the subjects it covers. It's designed to be a broad-brush overview, from beginning to end, to expose you to all the pieces, and give you a starting point for learning more about those pieces. For that reason, this book makes an excellent first book on Rails.