From my first days as a writer I was told to be very particular with the cover letters I wrote to editors. No joking, no remarks and keep the tone of the letter professional. And that is what I have done. I treat all other correspondence with editors in the same way.
I write all my letters in Times New Roman font and I go to great pains about the setting out of the page to make the letters as pleasant looking and as readable as possible. I use the Bold and Italic features as little as possible. I leave wide margins so the editor can make notes in them. If I have to add any titles I use Arial bold, which goes well with the Times New Roman and I never use a font larger than 12. I keep my sentences short and to the point.
As to the length of the letter, I try and make it as concise and as articulate as possible. I work on the basis that the editor is always very busy and will only devote a limited amount of time to each letter. So to make sure he or she gets all the way through my letter, it is crisp and to the point. I do my best to write a letter that conveys a little of my personality and a lot about my skills as a writer. After all, if the editor doesn’t like my cover letter, it is unlikely that they will like my article. If I can’t sell myself in 20 lines, I may never be able to sell myself at all.
I never use words and phrases that aren’t found in daily conversation. Just because your letter is professional, it doesn't mean that cannot be ‘user-friendly’. When you read your letter out loud it should be smooth and contain normal language and vocabulary.
Take a ‘less is more’ approach to your cover letter. Choose your font, keep it simple, no more than one page and print with black ink on white paper. You will not attract positive attention by using colored ink or paper. Cover letter writing relies on clarity and not color.
Your letter should include a short paragraph describing your track record in the literary field. You should also include a short summary that states why your work is unique and marketable.
Your may express appreciation to the publisher for the time they spend with your manuscript but don’t overdo this.
The hardest part is patience. It’s going to take time. It can take weeks, months or maybe a year to receive a response from the publisher. Do not call them and ask if they’ve read your letter yet. This will probably decrease your chances of acceptance.