Not a book, but scipy had a two-part introduction to Julia talk that's worth watching:

part 1: https://www.youtube.com/watch?v=vWkgEddb4-A

part 2: https://www.youtube.com/watch?v=I3JH5Bg46yU

There's not much in the way of books, but a few will be coming out soon, I believe. "Learning Julia" by Leah Hanson, for example. See also [1] for more learning resources.

[1]: http://julialang.org/learning/