Cover letters should be short and to the point.  Most editors only glance at the letter, maybe checking to see if they know you personally, then turn to the manuscript.  Cover letters will not get you published, but if you can succinctly describe what makes your manuscript particularly cool, then do it.  The key is to be succinct and professional (and to have a great manuscript).  Check for typos!

Here's the letter I wrote to Christy Ottaviano at Henry Holt and Co.  Christy e-mailed me two weeks later expressing interest.

Dear Ms. Ottaviano,

I hope you will consider the enclosed, preschool concept manuscript, Circle Rolls.  It is a bit of a metaphor for the way kids romp through the world ("Circle rolls") and moms patch them up ("Heart squeezes").  The story satisfies my childhood need to see Humpty Dumpty put back together again!

I am a member of the SCBWI and have an emergent reader, Mr. Miller's Old Car, published with Seedling Publications.  I also have stories and poems published in Highlights for Children, Ladybug, Guideposts for Kidz, Fun for Kidz, Story Friends and Wee Ones magazines.

Thank you for taking the time to consider my work.  I look forward to hearing from you soon via SASE.

                                                Yours truly,

                                                Barbara Kanninen