I sell children's books for a living, and read almost nothing but. The first does not necessarily cause the second, as books for children and teens are better than anything in the world.
Why did I wait so long to read this absolutely magnificent book? Why, why, why? Definitely upper YA (sex) and definitely crossover into adult. It's a futuristic fantasy set in a dystopian Brazil, and it's gorgeously written and features one of the most complex heroines of recent memory. Read this now!