The Query Letter
The query letter has a pretty straightforward structure and should take up no more than one page. In your query, be sure to include:
An introduction
Keep this brief. All you need to say here is: "Dear Ms./Mr. _________", and then say that you're seeking representation. Easy.
A summary of your book
Be sure to mention the title of your book, as well as the word count and genre.
Give a concise, engaging preview of your story.
This should NOT be a synopsis – don't give a blow-by-blow account of the events in your book. This part of your letter should read a lot like the blurb on the back of a book. It's meant to make the agent want to read the book, but it shouldn't reveal everything about the book.
Make sure to give the main character's name and a strong sense of the overall dramatic conflict. Also, let the agent know the general tone and style of the book (Is it comedic? Suspenseful? A tearjerker?).
NOTE: The summary is probably the hardest part of the query letter to craft. Writers are rarely good at summarizing their own work, so you might want to enlist the help of a trusted reader or friend to help successfully convey the highlights and strengths of your book.
A comparison to other recent books in the marketplace
Agents will want to know whether they can sell your book, so it's helpful for them to have a frame of reference for what you've written. Is your book a Neil Gaiman-esque fantasy? A book of linked short stories, similar to Olive Kitteridge? Give examples that relate to, but are not exactly like, your book.
Describe what makes your book stand out from other books like it. While agents want to know whether your book is marketable (i.e., what has sold before), they also want to know what makes it unique (i.e., what readers haven't seen before).
For example, The Hunger Games was a big hit, but agents don't want a story that's too similar to The Hunger Games – that particular story has been done before and won't sell well. So, if you've written a dystopian YA novel, describe what makes it different from other successful entries in the genre. Similarly, if you've written a memoir along the lines of Eat, Pray, Love, make sure to tell the agent what makes it different from that book.
A reason why you decided to contact this agent in particular
Do they work with a particular author you admire? Do they represent books that deal with subject matter and themes similar to the ones in your book? Have you researched them thoroughly online and think they'd be a good fit for your fiction?
Agents want to know that you've done your homework. If they say on their website that they don't rep genre fiction and yet you've sent them your sci-fi manuscript, your letter will be promptly binned. Make sure your letter shows that you know who they are and what kind of work they represent.
SIDENOTE: Triple-check to make sure you have the agent's name spelled right. Also, make sure you don't address a "Mr." as a "Mrs.", and vice versa. (If an agent has a gender-neutral name, do enough Googling to make sure you're using the right form of address – most agency websites have photos of the agents next to their names, so that's a great starting point).
An author bio
Tell the agent about your writing credentials, if you have any.
Don't worry if you don't have any! If you don't have any publications, degrees, or awards, feel free to say so. Agents will be primarily swayed by your summary paragraph and your sample pages – the fact that you don't have an MFA will not be the deciding factor.
However, if you do have publications, degrees, and awards, sum them up succinctly here.
You don't want to give your full résumé, just a highlights reel. If you've been published in nine online magazines and five print publications, just mention two or three of the publications you're most proud of.
If you have any work or life experience that reveals your credibility in writing about your chosen material, tell the agent about it!
Perhaps you've written a novel set in a prison, and you actually spent time in prison. Or, you're writing about horse ranching, and you spent your entire childhood on a horse ranch. These types of experiences pique the agent's interest and shows them that you have unique insight into the story you've written.