Briefly: What we're looking for in a query
First thing: You're going to write the main content of your query letter and send it to the very people you want to look at, and be impressed with, your manuscript. The people you are trying to convince to pitch it to the actual publishing industry on your behalf. The people who routinely see the very best from some pretty amazing authors. That's exactly who we are.
So. What you need to do is prepare your query letter with at least as much care and skill as you put into your magnum opus. Pay just as much attention to the grammar, spelling, pacing and so on in your query.
Next, don't go long. Make it one page-ish. Within that constraint, use your writing skills to get the following across to us:
Further, you need to work the above list in such a way as to seriously impress us within the context of the query itself. Every word you write matters, as does how you write.
Dictation, grammar- and spell-checking software can all introduce problems. Spell-checking software does not pick up errors when those errors are the wrong word, spelled correctly. It is also far too easy to select the wrong word to replace a word the spell-checking software has highlighted for you. Your intention is, of course, to type or dictate the correct word, but — particularly in the case of a dictation program — if a homonym is available, it can end up in your manuscript.
Queries and submissions should not be sent out to an agent or an editor without a thorough reading, not only by yourself but (at least) a first reader and even better, a second reader as well.
Next, for all email submissions provide just a few pages of the work you are asking us to represent as an attachment. For postal submissions, include the same as hard copy. They don't have to be the first pages, either. Something you think will catch our interest. Attachments should ideally be in plain text form. Send this to us with the query; don't wait for us to ask. If your query is well written and your subject matter interesting, we will look at the attachment or printout, and what we find there will most definitely count towards whether you hear back from us, or not.
Finally, you need to understand that we get a lot of queries. A lot. This means that there exists a long line of hopefuls ahead of you who got their work to us first. The only way we can really be fair is to examine each submission in the same order it came to our attention. So be patient. Craft your query with as much loving care as you can muster, send it along, and then... find something else to do and try to forget you sent it. If your work is really good, you may be assured you will hear from us: we'll be wanting to see the whole thing. Which will then call for another period of patience.
Hopefully you're getting a feeling for the process now. Nearly every step typically involves waiting — and then waiting some more. Waiting for the query process to produce results, working through any changes required, waiting for submissions to publishers, waiting for actual publication, and then waiting for the book to earn.
We wish you the best of luck.