Here's a book that does exactly what it says on the tin - teaches you how to write the perfect covering letter to go with your submission. Well not the perfect one perhaps, but a really good one that won't put off an agent and will hopefully entice them to actually read your submission.
I've read it quickly as I'm not yet ready to send out any novel submissions, but will refer to it no doubt when I am. I got the impression you could almost paint by numbers - include the paragraphs Nicola suggests, add your own details and twists, hone it and polish it and you're done.
This book and Write A Great Synopsis (both available for Kindle) should take away all the fear of writing these pieces, leaving you with more time and mental energy to focus on the important bit - your book itself.