A mystery novel query letter to a literary agent should include four single-spaced paragraphs in its 250-word entirety. But there's more that needs to be couched within those paragraphs than meets the eye -- the platform, to be exact!
Prior to Paragraph 1, insert a greeting line: Dear Ms. Greenbaum,
Paragraph 1: Two-sentence description of why you are querying that particular agent (met at writers' conference, researched that agent's genre interests on Internet, found in Writer's Market, referred by another author), along with the italicized title of your book and its word count,
Paragraph 2: Forty to fifty word description of your novel, i.e. introduce your main character and immediately share her/his problem/choice, along with who wants to foil that character's plan and why.
At the end of that paragraph, include your PLATFORM: list three themes in your novel that will spark interest in your readers, i.e. gardening, dogs, foreclosures, child abuse, terrorist attacks. You can also compare two authors, i.e. Dean Koontz meets Sara Paretsky, or compare two authors' main characters -- can't help you on that one. I stink at recalling character names. Think of it like Wonder Woman meets Willy Wonka. Of course any comparison you use must flow with the tone of your book.
Paragraph 3: Tell the agent why you are qualified to write this book, i.e. speaking engagements, special interest, career, hobby, volunteer activity, education degree in that particular subject area.
Paragraph 4: Check submission guidelines before e-mailing or mailing your query. If agent only asks for this one document, ask if you may send the first-three chapters or a proposal of your manuscript. Sign off by saying: Looking forward to hearing from you....
Then sign off with: