Here is a list of books I’ve found with information on wavy wefts. There are a number of ways to accomplish it.
Collingwood, Peter. “Two Weft Distortion Effects in Plain Weave”. The Journal for Weavers, Spinners and Dyers. Number 36, December 1960.
Collingwood, Peter, article above reprinted in: The Journal for Weavers, Spinners and Dyers, Peter Colllingwood Special Edition. Summer 2009.
Straub, Marianne. Hand Weaving and Cloth Design. New York: Viking Press, 1977. pages 86-87.
Sutton, Ann., The Structure of Weaving. London: 1982. pages 50-51.
Sutton, Ann, and Sheehan, Diane. Ideas in Weaving. Loveland, CO: Interweave Press, 1989. pages 104, and 97-101, special reeds.