It's often difficult to find good intermediate to advanced technical books that help you get over the hump. This is most definitely one of those books, IMO, along with Javascript: The Good Parts by Crockford and High Performance JavaScript by Zakas.
If you're a beginner, even an ambitious beginner, such as an experienced programmer in another language, you don't want to start here. For beginners, I'd recommend Zakas (Javascript for Web Developers) as the most complete introduction to Javascript, the DOM and browser scripting; or Simply JavaScript from Sitepoint for a gentler introduction that emphasizes the separation of structured content (HTML), presentation (CSS) and behavior (scripting the DOM with Javascript).
OTOH, if you're more or less comfortable with core Javascript and the DOM but want to clarify and explore the many idiosyncracies and fine points of JS, this book really hits the sweet spot. The table of contents is available on Amazon or O'Reilly, so I won't recap it - but will mention that Stefanov both chooses his topics and covers and organizes his material very well. This is a precisely and well-written book, and the reader will infer that there must have be a lot of experience, previous history and discussions behind these 200+ pages. I've read the blogs of the majority of his technical reviewers and believe you're in good hands here. I'm really lovin' this book. The only caveat - don't expect a lot on browser scripting. However, I'd be surprised if the somewhat experienced, but non-ninja, Javascript programmer did not significantly take his/her knowledge to a higher level after reading JavaScript Patterns.
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Addendum: I was interrupted by a rush job during my initial reading of this book and after returning to it now and rereading the early chapters and carefully reading several of the later chapters, I'm even more impressed by this book. The heart of the book, when it gets a little more advanced (functions, object creation patterns and code re-use patterns) has proved really valuable.