I was very impressed by the OOP design described in this book. I don't think it has much to do with OOP that I know in Java or C++ for that matter. The importance of polymorphic behavior and dynamic dispatch is clear to have very decoupled code base.
After reading the book, I went back to a project that I am building and did a huge refactor to introduce more abstractions to my code and mainly to work on useful interfaces that has clear signatures and can be extended by the users of my code in the future. I learn a lot from here and gave me lots of ideas about OO.
I was frustrated and looking for a book on CLOS that didn't have incomprehensible brevities and this book was excellent. It has a very modest tone of writing and avoids confusing the user. There are other less thorough and more expert-tuned intros to CLOS, but this one is for the absolute idiot like me who needs some decent intro to CLOS. I absolutely recommend it.
Assuming you already know Lisp this is a clear exposition of CLOS and how OO should have been implemented. Alas, the brain-dead single-dispatch Simula version took over the world. Sic Trasnit Gloria Mundi.