The query letter is an elusive, special kind of witchcraft. It must perform several functions: market your story in one neat package, use an attention-grabbing angle, and offer something that can’t be found elsewhere.
One of the first sentences will serve as the “hook.” This is the most commonly used term, although I prefer to think of it as the “bait” — that juicy nugget that will entice your prospective agent, not the sharp dangly bit that will scare them off. (Please do not scare off your prospective agent.) This is also marketing lingo, because that’s exactly what you’re trying to do: sell a concept.
That hook should give the agent reading it a sense of the story to come, but keep it at just enough distance that (s)he will want to tease the rest from you. Crafting this single sentence is perhaps one of the single most difficult tasks a writer must face.
If you asked me, “What’s your book about?”, the hook is often the follow-up line I use. You, while well-intentioned, likely did not understand how incredibly difficult that question was for me to answer. How to distill so much work into so few words?
Years ago, before having muscled through the query letter like trying to squeeze an artisanal espresso out of a rusty French press, I would have just gone off the top of my head. I would have babbled that my main character is a Strong Female Character (which is subjective even in modern literary discourse), that the plot is based on Ancient Roman mythology but not really (and even the Romans were derivative), or that it’s a serious book ultimately about werewolves (although the word “werewolf” isn’t used once).
“So…what’s your book about?” you’d then repeat, disappointed, but no more disappointed than I’d be at myself. Immediately, I’d go create a shame-space in the corner, sit there, and think about what I’d done (or in this case, failed to do).
As a writer, while your idea might appear fully formed and real as the world you inhabit, it’s intangible to those around you. The entire process of writing it, in fact, is shrouded in a veil of secrecy. Your readers won’t know the lengths of your research or the scrapped drafts that produced the end result, and that’s just fine.
But if your ability to summarize your project is only halfway there, others will inevitably stick their own assumptions on it. “It’s a fantasy, so it will involve dragons”; or, “it’s YA, so there will be a love triangle.” Maybe not in so many words, but it’s human nature to stereotype genre fiction, and we tend to be immensely critical as a mass media consuming culture.
Meanwhile, you (the writer) are in the background flailing your arms. “Wait! That’s not what it’s actually about!” you say. “My intention was to subvert those particular expectations while retaining some intrinsic tropes!” Your yelling is futile, because people are very good at believing what they want about something before being exposed to its true nature.
Your job, then, is to convince without hesitation, and that’s where the hook/bait comes in. It is witchcraft, and more specifically alchemy, because you are transforming that intangible idea, shaping it just so, into an object others can touch (and hopefully relate to, and understand, and even love).
“Don’t judge a book by its cover,” readers say, perhaps without realizing the books they hold were each given merit on the judgment of a single sentence.
Further reading on query letters:
Trying To Find a Literary Agent Is the Worst Thing Ever, Ken Pisani, Publishers Weekly
X Meets Y, or the High Concept Pitch, PubCrawl Podcast
The Complete Nobody’s Guide to Query Letters, Lynn Flewelling, SFWA