You want to write professional-grade applications: Rails is a full-stack, open-source web framework, with integrated support for unit, functional, and integration testing. It enforces good design principles, consistency of code across your team (and across your organization), and proper release management.
But Rails is more than a set of best practices. Rails makes it both fun and easy to turn out very cool web applications. Need Ajax support, so your web applications are highly interactive? Rails has it built in. Want an application that sends and receives e-mail? Built in. Supports internationalization and localization? Built in. Do you need applications with a REST-based interface (so they can interact with other RESTful applications with almost no effort on your part)? All built-in.
As with the previous editions of the book, we start with an extended tutorial that builds parts of an online store. And, of course, the application has been rewritten to show the best of Rails V2.