The genre freedom within the Young Adult market is one of the things I love about it, and one of the reasons why I now write Young Adult fiction.
'Young Adult' is, of course, an age category, not a genre in the way that 'crime', 'chicklit', 'fantasy', or 'literary' are. In fact, the entire Children's book market is subdivided by age, not genre. Which means you'll find historical novels shelved alongside horror, sitting beside high school romance, beside high fantasy. And, more importantly as far as I'm concerned, you'll find single books which offer a glorious mash-up of all four. And no-one minds.
There's always been such freedom in Young Adult fiction (and Children's fiction more widely) to throw together whatever bundles of oddness you feel like writing. It's immensely liberating for both writer and reader.
The book I'm currently reading is a good example. On the face of it, 'Cuckoo Song', by Frances Hardinge is a changeling-child story set in the 1920s, yet it's also gothic and fantastical and literary. If it was written for adults, who knows where it would be shelved, or whether it would even be published at all, or rejected as 'uncategorisable'. However, since it sits in the Young Adult age range, it has no such concerns. It can be precisely what it wants to be and still find an audience.
It's just as well, since MoulderingBook is a Gothic, Poe-esque story set in a historical period that never really was, with sort-of Steampunk nuances, and an Addams Family undertones (thanks for that contribution to the list readthisandweep!) Since it's for the Young Adult age range, its fluid identity doesn't matter. I'm free to simply keep writing and let it be what it wants.
I hope the Young Adult market doesn't ever feel the need to start pigeonholing by genre within itself. Long live genre freedom!