Kameron has some excellent advice for those of you trying your hand at writing query letters.
The biggest problem that I see in query letters - and that I used to see when I was a publicist training new publicists how to write press releases and pitch letters - is that they are just too damned long and never get to the point. All the good stuff? Put it in the first paragraph. Period.
And in a query letter, I want to know the entire plot of your book by the time I finish reading that paragragh. Because if I don't, I probably won't finish reading the letter.
Remember that, essentially, a query letter is a pitch. You need to be able to get your entire plot across in about two to three sentences. You need a hook at the beginning, something that makes me want to finish reading the rest of the letter.
If you can't organize your thoughts about your book concisely enough to write a clear pitch letter, it's probably indicative of your writing style as a whole. That's a red flag to agents.
A well-respected agent I know who has been in the business for more than twenty years recently said to me "Colleen, remember this: the writer never gets any better than the writing you see in the pitch letter."