Pro JavaScript Techniques

John Resig's, Pro Javascript Techniques was great. It covers good ground and from it you could see how to build something like jQuery yourself.

The JavaScript Anthology is good. It doesn't focus on the language details a whole lot, but gives tons of hands on examples. My copy is battered pretty bad with coffee spilled all over it. That's a good sign. Also, John Resig (of jQuery fame) wrote an excellent book called Pro JavaScript Techniques. I read it every year. It is a very detailed exploration of the language fundamentals and beyond.

It really depends what sort of hacker they are. There's a sort of hacker you buy a copy of "Pro JavaScript Techniques" for, a sort you buy the Dragon Book for, and another sort who you give a copy of TAOCP.