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We all know the first line of a query is your "hook." I call the last line the "sinker." You want it to punch them in the face, in a nice, friendly kind of way that makes them unable to forget you after having read the 300 other queries in their inbox.
If you're looking for query advice, but are slightly intimidated by my claws, blade, or just my rolling googly-eyes, check out the query critique boards over at AgentQueryConnect. This is where I got my start, with advice from people smarter than me. Don't be afraid to ask for help with the most critical first step of your writing journey - the query. My comments appear in green.
Nothing ever happens in the town of Stillman Valley, until their secret keeper disappears. Hmmm... it's definitely not a bad hook, because I want to know what a secret keeper does and why a town would have one. But I can also see that it could be a little too confusing and cause the reader to stop reading. Honestly on the fence on that one.
16-year-old Mags is sure her brother, Elliot, will show up in time for dinner. That seems like a weird thing to be sure about. Instead it sounds more like it's an assumed. Like I'm sure that the sun is going to come up tomorrow, but I don't sit around aware of my surety. I just assume it will happen. But when he doesn't, and when the search parties get lazy, Mags gets suspicious. How do they get lazy? Why would it make her suspicious? And (oops, don't start a sentence with and) starts a private investigation of her own. Even if it means this phrase usually is paired up with something... like, I'll eat this whole tub of ice cream, even if it means puking later. So there needs to be more of a connection between one thought and the next teaming up with that bipolar guy from school, AKA 'Creeper', who thinks he's a detective.
As they question friends and spy on neighbors, they discover just who Elliot really was. This phrase makes Elliot sound somewhat sinister. As the quiet eyes of the town, Maybe Elliot's quietness should be illustrated sooner? Elliot saw and heard things he shouldn't have, and was busy collecting expensive gifts in exchange for his silence. So he's blackmailing, but it still leaves the question of his personality out there.
But nobody likes their secrets snooped. And (you did it again) even Mags' parents will do anything to keep her out of trouble, are they keeping her out of trouble or protecting themselves? and the garage--whatever it is they're hiding in there.
Now that they're the ones being watched, by who, and how do they know that? Mags and Creeper must expose just how deep the corruption lurks in their small and twisted town, before they, too, disappear.
But what kind of corruption? And who do they think is watching them? You've definitely got what sounds like an interesting story, but overall if you look at it in detail it's actually quite vague, which is the problem. Boy sees things. Blackmails people. Sister goes looking, discovers things. Is in danger now. Has a cooky partner. This could be any number of books.
What specifically is being covered up? I don't know form the query. It could be environmental irresponsibility, sexual misadventures, animal abuse, corporate issues... literally anything. Details. Details. Details What makes this mystery different from all the others?
Also - I have no feel whatsoever for Mags personality. Elliot is quiet. Creeper is vaguely creepy and possibly unstable. What is Mags? From this query, I don't know. You also throw in the parents out of left field, which feels awkward and inserted. If she feels like she can't trust anyone, cool. But the parents are present for literally only the words that surround the word "parents."
And what happened to Elliot? He's the impetus and then suddenly it's more about the secrets and exposing them than finding Elliot. He drops out of the query after the discovery of him being the secret keeper - which, btw, that phrase itself made me think not of blackmail and secrets but of "keeper," like someone who is responsible for the well being of the town... it had a bit of a fantasy feel to it. That could be my reading of it, but using that phrase at the beginning made me unsure of genre at the start.
BLACK KEYS is a YA Mystery of 68,000 words. The theme of siblings but how important is the sibling connection? will appeal to readers of Lauren Oliver's Vanishing Girls and Jandy Nelson's The Sky is Everywhere, along with a light investigatory flair for fans of the Midsomer Murders TV series.
Good comp titles, just make sure it's true to what the ms is.