A Little Ruby, A Lot of Objects
Brian Marick started a book on objects in Ruby five years ago called A Little Ruby, A Lot of Objects [PDFs here]. Its written in the unique style the The Little Schemer and A Little Java, A Few Patterns. You could go as far as to call these books the Socratic method applied to learning programming.
Unfortunately, Brian only made it to chapter three, but the few chapters he did write are well worth reading. They really cleared up some my misconceptions about the object system in Ruby (though I still need to work thru the third chapter where everything goes crazy at the end). Recommended.