A query is a business letter. It isn’t your application to college. It isn’t the ONE thing that’s going to make the difference between publishing your book and not publishing your book. It’s a query letter that is presenting a business proposition to a business person. Yes, it should be well-written. No, you should not panic if you see a typo in it and immediately beg the agent to delete the old query letter and only read a new one. Because once you start on this process, you will do it fifteen times. Trust me, I know.
You should describe your book. Use genre words. Tell about how long it is. If you have relevant expertise, talk about that. Most of the query letter should be the middle paragraph, in which you use a nice hook (what makes your book special and why your characters are on the journey they’re on) and tell a bit about the adventure and the ending. Like you’re talking to a friend. A friend in the business.
I see too many writers obsessed with writing the perfect query letter. They seem to spend all their daily energy for a year or more on crafting the perfect letter. This is a misuse of your time. You should be spending most of your time writing your book. And rewriting it. And rewriting it again.
Agents mostly reject query letters because the book just isn’t for them, and no perfect query letter would change that. They also reject books because they aren’t good enough to publish yet. They may or may not be salvageable. I don’t know the answer to that from this distance. Not getting any responses to your query should be interpreted as a chance to rewrite the query and do the book better justice.
But more often, not getting any response to your query should also be a call to perhaps considering that the book itself needs major revision. Or that it’s time for you to work on a new book.
Good writing matters. Don’t get me wrong. It matters a lot. It’s just that it should matter in your actual book.