In addition to Andrews' The Gift To Be Simple and Patterson's The Shaker Spiritual, my collection includes:

Edward Deming Andrews, The People Called Shakers (Dover reprint). Classic.

Stephen J. Stein, The Shaker Experience in America (Yale University Press, 1992). History.

June Sprigg and David Larkin, Shaker: Life, Work, and Art (Cassell, 1987). Photographic book.

Candice Ord Manroe, Shaker Style: The Gift of Simplicity (Crescent, 1991).

Herbert Schiffer, Shaker Architecture (Schiffer Publishing, 1979).

Edward Deming Andrews and Faith Andrews, Work and Worship Among the Shakers (Dover reprint).

Rita Buchanan, The Shaker Herb and Garden Book (Houghton Mifflin, 1996)

Christian Goodwillie, Shaker Songs: A Celebration of Peace, Harmony, and Simplicity (Black Dog & Leventhal, 2002).

And a hymnal (not in front of me at present).