I started a topic here just this week. Please feel free to ask questions.
Summer & Fall Gardening in Charlotte
In regards to official sources, i would say the best source of information for planting gardens in the Southern Piedmont is the agricultural office at Clemson University in SC. They have information there that is specific to our climate here in Charlotte.
Home & Garden Information Center : Extension : Clemson University : South Carolina
Most gardening books, unfortunately, assume a less temperate environment than in Charlotte. As I indicated in my topic above, we really have to summer growing seasons as well as a very long fall season and you can start really early in the Spring. Good luck.