This isn't a book about heavy design patterns. Rather, it catalogs short recipes about how you can structure your object-oriented code to be manageable and readable — both very important qualities of good code. The book is written in Smalltalk, but given that Objective C is heavily inspired by Smalltalk, the concepts in the book carries over well.
Read the book and re-read it every year. It has been a great help to me and I'm sure there's a trick or two you can pick up from it.