I've started an ARC of I HUNT KILLERS by Barry Lyga, which isn't released in the UK until 12th April. I'd thought that it was an 'adult' crime novel but because the protagonist is the 15 year old son of a serial killer, who in turn begins an investigation after bodies start turning up in his home town, I've seen a number of people talk about it having YA cross-over appeal. I'm about 50 pages in and I can definitely see it working as a YA book so far - it's got a great teen voice to it and although it's obviously about murder, it's not unnecessarily gross. In fact, teens who watch CSI should be able to cope with it pretty well.
Anyway, figured I'd mention it here because I know a lot of posters here are interested in cross-over books.
MM