This is an older blogspot blog, run by one "Miss Snark." It comes highly recommended by several WritingForums.org members, despite not being updated since 2007. Despite not being updated, there's likely still some good information archived here.
Miss Snark's blog seems to have a lot of posts on "Agent Protocol," cover letters, first pages, writing hooks, writing synopsis's, and writing query letters.
Miss Snark - The Literary Agent
A blog with tips from an literary agent [last updated 2007]