"Practical Object-Oriented Design in Ruby. A Agile Primer" By Sandi Metz. I am not so much a ruby fan, but don't let the title decive you, ruby is just the tool (since it could use any other OOP language) to show what the book is realy about (OOP Design shown in the best way possible).
I think this book is a little too basic for anyone who's already studied any OOP. I'm no expert, but what I learnt from this could've been condensed into a blog post. I think it'd make a great introduction (a word that should be included in the title) to OOP though.
