Ruby's open-ended design makes it great for getting started quickly but very easy to write code that can't be maintained. POODR by @sandimetz is full of solid examples and best practices to enable you to build applications that start right and scale without breaking.
I love love love this book. I'm deep in it right now, and it's exactly what I need as a front-end developer/designer getting into construction of Ruby applications. This is probably the most recommended book in the Ruby community.
This book is awesome, I'm about halfway through it right now, but the information that I've already retained has changed the way I write Ruby for the better.