Your basic cover letter is such a simple thing: Get in, get out, and shut up. Here’s a no-frills model:
Enclosed are the first three chapters plus an outline of my 85,000-word science fiction novel, Voodoo Robot. It is [insert here a one- or two-sentence summary of the basic setup and story arc]. This is my first novel. [OR: I have the following publication credits.] [Optionally, and only if EXTREMELY pertinent: In addition, I have the following related credentials or experience. For example: This is my first SF novel, but on the other hand I am Geoff Landis.] I also enclose a self-addressed stamped envelope. [OR: You need not return the manuscript.]
Thank you for considering my submission. I look forward to hearing from you.
[your name here]