alistapart.com/articles/sensibleforms

A great set of guidelines for designing online forms.

Barnett, Robert. Forms for People: Designing forms that people can use. Canberra: Robert Barnett and Associates Pty Ltd., 2005.

Jarrett, Carolyn and Gerry Gaffney. Forms that Work: Designing web forms for usability. New York: Morgan Kaufmann, 2009.

Johnston, Carol, Ginny Critcher, and Ellis Pratt, How to Write Instructions. Cherryleaf, 2011.