The cover letter is the introduction to your book proposal. It is a formally written business document rather then a ‘fluff’ piece. It should only be one A4 page long.
At the start of the cover letter you should greet the publisher or agent and spell their name right.
Have a pitch that is either twenty-five words or less. This should be a strong sentence or two that states the concept of your manuscript. This is not the place to explain the whole plot line of your story.
State why you are the person to write this book. If you don’t have any qualifications in the field (the non-fiction topic or in writing), then state how passionate you are about the genre or topic.
In bullet points have a list of what is in your book proposal (like a mini contents page).
Say what stage your manuscript is at. Is it finished? Halfway done? Make an estimate of when you think you can finish it by. It is always better to overestimate how long you need rather than underestimate.
The last thing on your cover letter should be a closing statement and, if you sent your book proposal in threw the mail, an SSAE (Stamped, Self-Addressed Envelope). You want to do everything in your power to get the publisher or agent on your side so paying the return fee is the least you can do.