Now that I've shared my letter, I'll offer a few general tips on writing the best query letter for your project.
1. If you have a personal connection to this agent, lead with that. You may have friends who are already represented by literary agents. By all means, ask them if you can drop their names when you query, and don't bury this information. Begin your first paragraph with, "[Author name] speaks very highly of you and suggested I contact you about my novel..." et cetera. Remember that whoever is sorting through these emails first is reading dozens at a time. If you make it clear that you have a mutual acquaintance, it's much more likely your query will receive further consideration and a prompt reply.
2. Follow the agent's instructions. I can't stress this one enough. Almost every agency includes detailed querying instructions on their website; follow these to a T. Sometimes they even vary for the different agents in the office. Some will want the first five pages of the manuscript pasted in the body of the email; some will want an attachment. Some will want only the query. If the agency will only take snail mail queries, do not hunt down an email address and email them anyway. And by all means, do NOT call to "discuss" your book. You wouldn't call a potential employer to discuss your career options. You'd send a cover letter asking for an interview, then wait. That's what the querying process is: your letter lets them decide whether they want to request the manuscript, and their review of the manuscript is the equivalent of the interview.
3. Personalize your letter. Why are you approaching this agent? Is it because she represents authors of historical fiction and you think you'd be a good fit for her list? Does she represent authors you admire? Did you just love the welcoming message on her website? Say why you're querying him or her, and make sure you get the name at the top of your email correct. Mass emails or cookie-cutter messages labeled "Dear Agent" will get you nowhere. An exception to this is contained in my second tip: there will be a few agents who don't want you to waste any time, who specifically say they only want to hear about your book and you, no fluff. There aren't many who say this, though, and I think it's best to generally personalize your letter.
4. Read the backs of your favorite books to help you shape your pitch. Professional book designers and editors know exactly how to draw customers in with a few salacious lines. This is exactly what you need to do to get an agent's attention. Go back to your favorite books, or the ones that took you from the New Releases table to the checkout line in Barnes & Noble, and carefully dissect the copy on the front and back covers and inside flaps. Then try to replicate the same kind of language about your book. Still stuck? Ask a friend who is familiar with your work to help you boil it down to the essentials, the few details that make your work unique and would leave an agent wanting more.
I could probably keep going, but I'll leave it there. I am always happy to hear from writers and will take query questions (is that redundant) anytime on Facebook, Twitter, or the contact page of this website.
Good luck with your query!