Be warned, Unity’s javascript is not true javascript as you’ll find reference for across the web. The syntax and semantics of the language are almost exactly identical, but you’re dealing with a set of objects provided by Unity and the Mono Project.
For instance, theres no alert(), instead, there is Debug.Log(). An extremely large part of any javascript reference is really focusing on the language’s usage in web pages, which don’t apply in Unity itself.
If you’re not into huge books, you might consider at least picking up a copy of the JavaScript Pocket Reference from O’Reilly. Unlike it’s parent book, the JavaScript Definitive Guide, the Pocket Reference is just enough to remind you about the most commonly used features of the language and the proper syntax for calling upon them when needed.
Now if only we could get a proper printed manual for Unity…