American Architecture: Sandra Pollock provides a clear outline of the development of styles from Georgian to American Foursquare, illustrated with examples.
Architectural Styles Index: Created by Ingolf Vogeler of the University of Wisconsin as a guide to styles in the American Midwest. He gives the distinctive features of each type, illustrated by an example. Bibliography.
Architectural Styles of America: Thomas Paradis of Northern Arizona University supplies sample images and descriptions of the dominant architectural styles found throughout the United States.
Susan's Victorians: A photograph gallery of Victorian homes from around the United States, arranged by style, with commentary.