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.
The Pacific North West Sacred Harp Singers website has a list of links to useful resources.
FaSoLa Minutes is an iOS app that lets you browse and search the Minutes of Sacred Harp singings and listen to recordings. It’s $4.99 and all proceeds from sales are donated to the Sacred Harp Musical Heritage Association.
FaSoLaMix is an iOS app designed to teach Sacred Harp singing to newer singers. The app comes bundled with recordings of 24 songs, ranging in difficulty, all of which are regularly called at all-day singings and conventions. More recordings will become available for sale periodically in the form of “song packs” which will generally include several songs. It’s $14.99.