Since A Little Less Girl features teenage characters and specifically tackles a whole bunch of issues that young folks might relate to, it's generally classified as young adult literature. That doesn't mean that old adults can't enjoy it, though, so read away, thirty-somethings. No judgment here.
But this book isn't just hanging out on YA shelves. This tale also contains elements of mystery. See, everyone in town thinks Amy committed suicide, but Dani just doesn't buy it. She turns into a bit of a detective as she pours over Amy's writing and artwork to find clues that will tell her how and why Amy actually died. And even though Dani comes very close to accepting that she'll never crack the case, in the end she finds the culprit and Jake is sufficiently awed by her investigative powers.