Hey, there. I have plenty of advice on the subject of writing, being a writer myself (unpublished, but I have several works in progress). Absolute best book I can recommend on all aspects of writing is "On Writing" by Stephen King. It's utterly fantastic and that book changed my perspective on the craft forever.
Some books for writers specifically about character description are:
Elements of Writing Fiction - Characters & Viewpoint (Elements of Fiction Writing) by Orson Scott Card
Characters, Emotion and Viewpoint: Techniques and Exercises For Crafting Dynamic Characters and Effective Viewpoints (Write Great Fiction) by Nancy Kress
Creating Characters With Personality: For Film, TV, Animation, Video Games, and Graphic Novels by Tom Bancroft and Glen Keane
Writer's Guide to Character Traits by Dr. Linda Edelstein
Creating Unforgettable Characters by Linda Seger
Creating Character Emotions by Ann Hood
The Force of Character: And The Lasting Life by James Hillman
Getting Into Character: Seven Secrets a Novelist Can Learn From Actors by Brandilyn Collins
As far as websites go, I have plenty of suggestions. Try:
On this page we see what a real author thinks about what is most important in character description:
These pages feature excellent discussions about the same question you just asked.
And this is a great exercise to go through to help you organize your thoughts on how you will describe your character:
Also, if you need additional help please feel free to shoot me an email. I love writing and everyone that I know says I'm very good at it. Character description is one of my specialties. And I love talking about writing, I could go on about it for hours. So just let me know if you would like me to preview something for you or give you ideas on how a particular character could be described. :)
All above listed websites, plus amazon.com