JavaScript: The Definitive Guide is just fantastic. It is the only JavaScript book I've seen which clearly separates the definition of the language itself (covered in Part I) from the classes used to embed JavaScript in Web browsers (covered in Part II). It also covers many subtle points of JavaScript programming very clearly, including prototypes, the scope chain, nested functions and garbage collection. Furthermore, the book points out a number of bugs in existing browser implementations and has an excellent reference section. I can't recommend this book highly enough for anyone who already has a fair amount of experience programming in other languages and wants to learn JavaScript.

If you are learning JavaScript as your first programming language, however, this is probably not the best book for you, since the writing style is probably too technical for beginning programmers and the book pays a lot of attention to subtle details which may be overwhelming to those without programming experience.