YA books for adults

The line between YA and adult books is, for me, very blurry. And apparently for some publishers, too, which is why (for example) The Curious Incident of the Dog in the Night-Time is only one of many books published in both categories. Now the School Library Journal blog (link from Kids Lit) takes up the question, listing nine books for teens from the last year that would be equally at home in adult reading rooms. I've only read two (The Graveyard Book and Octavian Nothing, II) but I wholeheartedly agree about them. Then again, I'm an adult who routinely reads YA fiction, so I would, right? And Pratchett (another listee) and Gaiman already cross over all the time between children's, YA, and adult fiction, so they're no surprise. My husband thinks Octavian Nothing is categorically not for teens (we have these arguments periodically; they're fun); it is, certainly, a denser read than many books we typically think of as YA. But now I need to read the rest of the books on the list and figure out why they're there and not, say, Paper Towns, which was one of my favorite YA books of the year and far more engaging than, say, Prep, which also has a teenage protagonist but was published for adults.*

Categories are hard. Necessary, I guess, but hard.

*One (cynical) reason is marketing. It's also true that Paper Towns has few if any interesting and well-developed adult characters--they're just much less important to the story--while The Graveyard Book and Octavian Nothing each has several. For my money Prep** has no interesting characters of any age, but that's maybe just me.
**Yes, I'm aware that Prep didn't come out this year and the other books I'm talking about did. I couldn't think of a more recent "adult" fiction title with a teenaged protagonist.