After years of writing miserable JavaScript with only the ECMA standard and the DOM specifications to guide me, I finally decided to buy the O'Reilly JavaScript book—because I stumbled across it in a bookstore (actually a French translation, but O'Reilly books are usually well translated), and because I know that David Flanagan writes very good books.
I still didn't have much occasion to do more than glance through the book (I bought it yesterday), so I still can't say what it's worth. It does seem a bit outdated in many respects (everything that concerns Netscape 4 or lower, for example, is just about completely useless now, I guess); but I hope it will be useful anyway. I'll probably buy something Mozilla-specific as well. But I hope the book will inspire me, at least, some interesting uses for JavaScript.