Practical Object Oriented Design in Ruby is a book written by Sandi Metz which was recommended to me while I was working at Powershop. Amazon 
It's a great book that takes revisited concepts I already knew in OO, but from a Ruby perspective. I really appreciated the books tone and simplicity.
As the title suggests, it's very practical. The follow-up to this by Sandi and Katrina was 99 Bottles of OOP, which I'd also recommend.