WorshipWeb—an online resource for Unitarian Universalists.
The Congregation of Abraxas Worship Reader—includes several writings by Unitarian Universalist ministers.
Arnason, Wayne, and Rolenz, Kathleen. Worship That Works. Skinner House Books, 2007. A guidebook for revitalizing our worship life, Worship that Works provides practical, specific advice to improve the key elements of the service and increase the spiritual resonance of worship through symbols, music, cultural sensitivity, inclusion, and more.
Driver, Tom F. The Magic of Ritual: Our Need for Liberating Rites That Transform Our Lives and Our Communities. HarperSanFrancisco, 1991. This book is not so much a "how to" as it is an exploration of "why"—why worship can be, and should be, so much more than a concert and a lecture.
Neufer Emswiler, Thomas and Sharon. Wholeness in Worship: Creative Models for Sunday, Family, and Special Services. Harper & Row, 1980. While definitely Christian in orientation—the authors are United Methodist ministers—this book can show Unitarian Universalists a thing or two about creativity.
Proctor-Smith, Marjorie. In Her Own Rite: Constructing Feminist Liturgical Tradition. Abigndon Press, 1990. When this was published, it was considered a landmark in the study of worship from and within a feminist perspective. While Unitarian Universalists have done much in this regard, there is always more to learn.