You may want to read
The Mood Cure by Julia Ross. Even though it's mainly about improving emotional health, there's quite a few pages devoted to ADD.
I also recommend the short video "Nutrition & Behavior" by Dr. Russell Blaylock:
Nutrition & Behavior (MD Russell Blaylock 2006) Video
He talks about how important nutrients like DHA, zinc, magnesium, selenium and the B vitamins are to brain function, and how toxins and additives like food dyes, pesticides & herbicides, vaccinations, MSG and aspartame negatively affect the brain.
He also recommended eliminating sugar, including fruit juice like the boxed kind, soy (73% of hyperactive children in one study were sensitive to it), cow's milk (I assume industrially-produced dairy), and chocolate.
It's only 48 minutes long, but it's packed with information.
ETA:
I found a blog post that summarizes the video pretty well:
Dr Russell Blaylock – Nutrition and Behavior « A Phil-for-an-ill Blog