Most of the classic prayers are in the public domain. The books listed below are the primary sources.
Use or quotation of a resource is not neccessarily an endorsement of everything found in that book or website.
Ancient Collects and Other Prayers Selected for Devotional Use from Various Rituals, Ed. by William Bright, D.D., J.H. & Jas. Parker, London, 1902
The Catholic Prayer Book, © 1986 Servant Books, Cincinnati OH
Christian Worship: A Lutheran Hymnal, © 1993 Northwestern Publishing House, Milwaukee, Wisc. U.S.A.
Luther’s Catechism, Gausewitz edition, © 1956 Northwestern Publishing House, Milwaukee, Wisconsin
Luther’s Catechism, Kuske revised edition, © 1998 Northwestern Publishing House, Milwaukee, Wisconsin
The Lutheran Liturgy (St. Louis: Concordia, 1945?)
A Lutheran Prayer Book, ed. Doberstein, © 1960 Muehlenberg Press, Philadelphia
Minister’s Prayer Book, ed. Doberstein, © 1986 Fortress Press, Philadelphia
Mozarabic Collects, Translated and Arranged from the Ancient Liturgy of the Spanish Church, Rev. Chas. R. Hale, S. T. D. New York, James Pott, Publishers, 1881
The One Year Book of Personal Prayer, © 1991 Tyndale House Publishers
Seed-Grains of Prayer, A Manual for Evangelical Christians by William Loehe, Lutheran Pastor, translated by H. A. Weller. Chicago, Wartburg Publishing House, 1914
The websites are of varying levels of quality and varying doctrinal viewpoints. The websites were still helpful to confirm the sources, and to find other (and sometimes longer or better) versions of prayers.