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.
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Supply hunter Gorin absolutely despises his life in Middleton, Pennsylvania a hundred years after the plague devastated the world. Your first sentence here is pretty convoluted- you're jamming a lot of information into it and it just muddies the waters and raises questions. Two months from his seventeenth birthday—and certain death thanks to the birth-transmitted disease—his job is to find Valuable Objects and deliver them to his faction home’s leader. I'm definitely intrigued by the idea that their all destined to die on their 17th birthday. This has my attention more than the idea of someone looking for stuff in PA after the end of the world. There's a lot of post-apoc lit out there right now, so put what makes yours different up front. If only he could spend his last days studying those VOs, and just once lay his eyes on The Middleton Rulers’ mysterious mansion before he dies…But no—both are strictly prohibited and punishable by death. Slight question here in that if he's the one who finds the objects, doesn't he have at least a little time to study them before handing them over?
So when Gorin fails to win his last chance like this is something that is available even though it's punishable by death? to go on a tour of the manor, he and his friend Marf sneak a peek at the incredible white-marble building and make a terrifying discovery. The manor has Power, an almighty force that made life way easier back in the Old World. Clearly, the rulers have been lying to the rest of Middleton about their reasons for trying to rediscover the mystical force. And there’s no telling what other vital secrets they’ve also been hiding. Worse yet, Gorin and Marf got caught in the spy act. Now with massive bounties on their heads, they try to break into the heavily guarded mansion to gather evidence of the rulers’ deceit and save everybody in town from their tragically short lives.
I definitely need to know more about what Power is, right now it seems very ephemeral and I'm unsure on how it can save everyone from their short lives, let alone how a simple glance at the building reveals that it has Power. You also make it sound like the Mansion itself is the holder of the Power, not the people within? Also unclear is what Valuable Objects are, exactly, and how they contribute to the Power. Re-read your query as if you didn't know anything about the plot of your own book, and you'll see that a lot of it is veiled.