Ask HN: Any good JavaScript books?

I've seen the cool things that developers are doing with JavaScript. Especially http://sonspring.com/journal/jquery-desktop.

I'm wondering what the best book on the subject is. Just two things:

- It has to be freely downloadable online - It has to have assignments at the end of the chapter

As for the assignments thing, I think it's really great. It's the best way to show yourself that you learned what you read.

Oh, and if it matters, I know Java and Python.

Edit: formatting