In this small and dense book you will find JavaScript guru Douglas Crockford's presentation of the beautiful subset of a language that lies within JavaScript as a whole. You can use this subset to write serious programs. Occasionally you might have to use some of what Crockford calls "the bad parts", but at least you'll know the dangers, and how to mitigate them.
Perhaps more importantly, Crockford is very, very aware of how JavaScript differs from classical OO languages, and how these differences can and do trip up classically trained programmers, coming from (say) C++, C# or Java backgrounds. Crockford goes out of his way to point out the differences and the new way of thinking that is required for JavaScript.
This book is not necessarily going to be an easy read, even for professionals. Crockford says so himself in the preface. It's dense and terse. By necessity it introduces terms that might be confusing at first, and which are only explained later. You have to read the whole book, patiently waiting for certain concepts to be explained, and for things to fall into place. On a re-read it all begins to make real sense. I do recommend that this book is read at least twice, preferrably three times. At least it's short!
There is not much to do with the web in this book. There's no DOM manipulation examples, and no Ajax calls. You will find no discussion of modern JavaScript libraries. Crockford just focusses on his area of expertise: the JavaScript language. He highlights and promotes the good parts, and in appendices talks about the "Awful" and merely "Bad" parts.
This is an important book. Crockford writes with the authority, seriousness and simplicity of K&R. To be a modern web developer you really need to know what you're doing with respect to JavaScript. Look to other books to discuss JavaScript libraries, the DOM, animation effects and Ajax. Look to this book for the starting point to all that: the JavaScript language itself, weird and wonderful, familiar yet strange, bad parts and (fortunately) good parts.