I found Nathan Bransford's blog incredibly informative when I was constructing my query. He explained the
psychology behind the format, which was exactly what I most desired to understand.
He was formerly an agent an Curtis Brown. Now he's focusing on his own writing.
Check his blog out here:
http://blog.nathanbransford.com/
Basically, the format I used was:
Dear [Mr. Agent],
[Some ego-petting paragraph that explains why you are so eager to query him, usually by bringing up an interview he gave or a book you read and loved that you know he represents, and perhaps likening your own project to that book. Bransford stressed how personalizing a query right off the bat endears the agent to you and helps him take your pitch more seriously.]
[Pitch of book, taking up one to two paragraphs. Includes title of the book (in caps) and the word-count and genre.]
[Paragraph briefly mentioning your qualifications for writing this book. Where you graduated from, what you've studied, whether you've published anything else. If you have no qualifications, this is your opportunity to inject some humor into the query. Agents like funny, charming people.]
Thank you for your consideration.
Best regards,
[Your name]