For all of you new (or not so new) authors out there, if you're having trouble with your query letters, want advice on form and style, or are just looking to get a bit more into the writing world, I would suggest you check out this blog: http://blog.nathanbransford.com/. Read the posts and the threads, they are really helpful. Nathan Bransford is a real live agent with Curtis Brown and offers this blog for advice - and it's good advice. Today he is going to do a critique of a query letter from one of us. (I know. It's pretty awesome that he does this; it really helped me out.) He also has some great links on previous posts and page critiques submitted by authors. I definitely recommend checking it out, joining, submitting, and really getting into it.
Some more helpful links:
http://kidlit.com/2010/05/03/know-your-category/ - Mary Kole (Agent) on children's lit
http://www.scbwi.org/Default.aspx - for anyone who writes for children, the society of children's writers and illustrators
http://www.how-to-write-a-novel.net/index.html - I know it sounds dumb, but there is a ton of information on plot and character development, style, and the publishing process; pretty much anything you can imagine. And on top of all that, it's pretty well-organized.
There are a ton of resources out there, I've just listed a few that I found helpful. If you have more, please, please let us all know!