Agent Jill Corcoran has posted a complete list of what an agent (and probably an editor) wants to see in a query letter. Click on over to see.
Her blog is chock full of great help for writers.
One of her more recent blog posts is all about how to write first chapter books -- you know, like Clementine, Mercy Watson, Marty McGuire, Captain Underpants, etc. Those step-up books longer than easy readers but shorter than middle readers.
Edited and added:
Yet another post about writing a query letter, this time from Jane Friedman.