It didn't take long to realize that it was intended for young adults. The heroine is 15, for starters; and all the drama revolves around different cliques at school, staying out past curfew, and getting in trouble with parents who don't understand what it's like to be an almost-grown up in wartime NYC.
And, despite the title, it turns out it isn't a murder mystery. (The term "murder" is 1940s tween slang equivalent of "the bee's knees", which the reader learns almost 2/3 of the way through the book.) The story is about the disappearance of a high school kid from the Lower East Side, whose school the heroine just transferred to from her posh private school on the Upper East Side after her mother dies and her father is injured at Pearl Harbor. Her dad is private investigator hired to find the missing boy, but because he won't let her help him with his work, she goes ahead and does it anyway. Teenage drama, heartbreak, and rebellion ensue.
As a story about teenagers navigating their independence, it's not a bad book. It feels like it might become a series: a lot of the sub-plots aren't wrapped up, and a lot of tiny questions about characters aren't answered. As a mystery, though, there wasn't much to keep me reading. The end was a little anti-climactic, and the heroine doesn't really have anything to do with solving the case.
What the book did a good job of capturing, though, was that awkward in-between phase of the teenage years, where high school is its own social realm of mini-cultures and where adults just don't get how clueless they are about how capable and responsible teens can really be if they'd only be given opportunities to be independent. That adolescent mentality pervaded the book so naturally, it was brilliant.