Design Patterns in Rubygoodreads
Less formal and easier to read then GoF Design Patterns, with many nice examples. And what is even better, it focuses on Ruby! It also includes many details on Ruby itself, which is great when it comes to the “tricks” you forgot about or never new, but mostly it just wastes your time. Better place for those would be in appendices.
I also noticed some errors in the examples code (ie defining attr_reader when clearly attr_writer is what is needed) and also some strange ways to write Ruby code, but all in all this is a great book.