Robert Schenk, PhD
More About Demand
This essay provides a good description of normal goods, inferior goods, substitutes, and complements, and how these are each related to the law of demand.
Wikimedia
Wikipedia: Giffen Good
This site explains the conditions that must exist in order for a good to be classified as a Giffen Good. It also gives examples of times when a good could become a Giffen Good, e.g., potatoes during the Irish Potato Famine.