For an introduction to the principles behind WAC programs, a great place to start is with these important articles:
Why We Teach Writing in the First Place, by Toby Fulwiler
High Stake & Low Stakes in Assigning and Responding to Writing, by Peter Elbow
The City University of New York: The Implementation and Impact of WAC/WID in a Multi-Campus U.S. Urban University, by Linda Hirsch and Dennis Paoli
Also visit the WAC Clearinghouse website for further information and resources.