The use of Javascript in Unity (also commonly called UnityScript) is not the same as Javascript programming outside (e.g. making HTML5 games, WebGL, or using some other API/framework). So you don't want to get any Javascript game programming book.

There are a number of books on Unity, though some may choose to cover one of the other languages Unity supports for its scripting (UnityScript/Javascript, Boo, C#); so you'd just need check before hand. Though Unity's scripting documentation uses for the most part UnityScript/talks about the features in that context. So you can easily just check out their documentation.

Or if you have no prior experience with Javascript, you could learn Javascript outside of Unity; again just get a book focusing on that. And then you'll find it very easy to pick up the additions/changes Unity makes to their implementation, as well as how to go about using it just from their documentation (at least that's how I found it, as I was already knew Javascript before I used Unity, so their documentation was adequate enough for me to know how to use it with their engine).