Most of them are awful.
They are turgid and rambling, overwritten, overstuffed, and self-conscious. Most of the ones I've seen look as if the writers don't want to succeed, that they set out to write the worst possible introduction to themselves and their books. But I know that's not the case. It's your inner evil zebra (read Garth Stein's
THE ART OF RACING IN THE RAIN), coercing you to self destruct.
It's not that hard, guys.
What can be difficult is distilling your book description down into one paragraph - because that's really what you need. You need a hook, something that catches an agent's attention. Brainstorming with a friend can help immensely, because you're so close to your own manuscript you may not even recognize your own hook or the book's essence - plus you're going to want to cram in details that aren't really essential. Yes, they make your book better,
but they don't necessarily make your query better.

The rest of the letter is simple - Nathan Bransford
outlines it succinctly and offers
details and
samples. Janet Reid, aka
Query Shark, does a wonderful job of pointing out what's wrong with queries - I'd advise reading through most of these. And for fun, check out
The Rejectionist as well.
Here's my basic outline:
First paragraph: Why you're contacting this particular agent.
Second paragraph: A brief description of your book - one that
captures its essence. You probably don't need character's names or where they live, unless relevant to the story. You can stretch this to two paragraphs, but less is almost always more. Agents read a lot. They're tired. Entice them, don't browbeat them. I described my entire book in three sentences, the first of which was
A childless woman living in a small Adirondack town dives into icy Lake Champlain to rescue a young French-speaking boy. And trust me, every word in that sentence is there for a reason.
Third paragraph: Your writing or publishing credentials, if any, or your particular expertise in writing this book. Keep it brief, and skip this if you have done. Never say
This is my first novel or
I have no writing experience. Here I also mentioned that I'd lived and worked in the towns my novel is set in.
Fourth paragraph: A brief thank you.
Never say I will be eagerly looking forward to your reply. Just don't. I favor a simple
Thank you for your time.
And paste in the first few pages of your manuscript, even if not asked for.
(Never, ever attach anything to an email or ask an agent to click a link to your blog, although you can include your URL in your signature line.)
I had the very good fortune of having some complimentary comments from Famous Author and Well-known Editor (the payoff from attending a
writers conference) and quoted them near the top. And every agent who responded - except Molly Friedrich, who sent a polite letter of disinterest after I'd signed - asked for a partial or full (I cut the hunt off after three weeks).
When you're done, print it out and edit it (you'll find mistakes and poor phrasing on paper that you won't catch on screen). Then read it aloud. Edit again. If you know someone good at query letters, ask that person to critique it. Email it to yourself to make sure the formatting works. Imagine it in an inbox with hundreds of other queries and a bleary-eyed, overworked, frustrated agent. (See why I advise brevity?) Is it professional, engaging, friendly, clear, and succinct?
Does it make you eager to read more?
Now and only now are you ready to send it.