Literary agents can be very specific in how they think a query letter should be pitched. Send a query letter of no more than two pages, which includes your credentials, an explanation of what makes your book unique and special, and a synopsis.
Rather than leading with the plot, lay out the case for your book in a crisp, tidy four-paragraph format that begins: 1) Here is a (describe type of book), 2) It’s the story of (give only a three-sentence summation), 3) Here’s how the book came to be written and what people think of it, 4) Here are my credentials.I prefer a short, clear letter rather than one that is overwritten or opaque. By which I mean, get to it: Know how to talk about your work succinctly.
Others aren’t as encouraging: No unsolicited queries accepted.