The book puts a lot of emphasis on performance and efficiency, especially when it comes to scope, memory management and algorithm complexity. You will finally learn and understand what closures are all about. You will know how some statements work in some browsers (IE is always the slowest browser.) You will learn a ton of stuff you won't find anywhere else neither online nor in a book.
There is also a section on best practices including maintainability, performance and deployment that I found especially useful.