Sandi Metz is a programmer, teacher, author, and sometime consultant. In the past 30+ years she has written innumerable applications and creates practical solutions that produce working software that is easy to change. She has spoken about object-oriented design and refactoring at international Ruby conferences since 2009.
It's mostly about reducing object coupling and is full of example code. If you occasionally find yourself forced into a corner where you have to write case statements that switch on class, have a look, this will help.
Plus, the last two and a half minutes are full of bicycles; what's not to like?