Notes on: Practical Object-Oriented Design in Ruby by Sandi Metz
This book doesn't have exercises but there is a lot of code to follow along with and to try out. This book has been referenced in so many conferences and computer meetups I have attended and it is a good read.
Even if you don't understand it all; it is plainly written. You can always go back to it again later with a deeper understanding like you would with many classic novels.
:w