Wikipedia has a good overview of Sacred Harp wikipedia.org/wiki/Sacred_Harp.

www.fasola.org is a comprehensive site maintained by the Sacred Harp Musical Heritage Association.

originalsacredharp.com is maintained by the Sacred Harp Publishing Company. They also publish a newsletter and books.

The Pacific North West Sacred Harp Singers website has a list of links to useful resources.

Sacred Harp and Related Shape-Note Music Resources is a large collection of resources on shape-note music.

FaSoLa Minutes is an iPhone app that lets you browse and search the Minutes of Sacred Harp singings.  It’s $4.99 and all proceeds from sales are donated to the Sacred Harp Musical Heritage Association.