Gaiman for a 12 year old

My nephew is really into http://www.the39clues.com/ . Since his birthday is coming up in July I thought I'd get him one of the books or card sets, but my guess is that he's got them all.

A post in http://www.wired.com/geekdad/2009/06/3-great-audiobooks-for-kids-and-adults/ made me think of Neil Gaiman. Since nephew is into sci-fi/fantasy stuff, but is *just* getting into being an avid reader I thought he may like some of his 'meant for kids' kind of stuff.

I've read the Sandman series but haven't ventured into other Gaiman material. Can you suggest something that a mystery/fantasy/sci-fi interested 12 year old boy could get into? I'm not sure if the kid stuff is too young for him (I know for sure that the Sandman stuff is too old for him).

Thanks in advance! :)

Good Things

A lunchtime walk in nice weather.
Awesome homemade lentil beef stew for lunch.
Plans to go to Hampton in a couple of weeks.
Getting things done at work, and doing them well.
A night to myself to sit on the porch, watch some movies and just be with the cats.