If you haven’t seen this yet, from BubbleCow.net:
Dear [put the name of the agent here. Avoid using general terms such as sir/madam. Do your research and find the name of the agent that will love your book.]
[Elevator pitch -- A couple of lines that capture the essence of the book. This is a concise and targeted summary of the book in just a couple of sentences. e.g. Using the Three Little Pigs story the first paragraph would be... With his brothers already devoured by a serial killer known only as The Big Bad Wolf, the third pig fights for his life with just a pile of bricks between him and certain death.]
My book is called [your book's title in italics]. It is complete and consists of [word count e.g. 80,000 words]. My book fits firmly into [your book's genre - pick one genre only]. Readers of my book would also read [list three books that are in the same genre as your book and would be read by readers of your book. This will show that you understand the genre and will reassure the agent that your book is in a genre that they represent].
[This is a very brief description of your book's plot. The key is to keep it short. For the Three Little Pigs we get... Set in the fairy tale land of medieval Europe, this tale sees three pigs leaving home to make their way in the world. However, their coming-of-age is overshadowed by the presence of the serial killer known as The Big Bad Wolf. The first pig opts to build a house of straw, but it offers little protection from the Wolf. The second pig tries sticks, but with the same results and he ends up as the Wolf’s second victim. The third pig develops a plan that involves a brick house, a chimney and a pot of boiling water – but will it be enough?]
[The last paragraph is your biography. It should be short but include the following: 1. Any published books 2. Your writing experience 3. A bit about you 4. Anything unusual that may set you aside from the crowd.]
This is a great starting point, but remember to add your own flair! Ready. Set. Query!