Gregorian chant

Gregorian chant is also known as plainchant or plainsong, and is a form of monophonic, unaccompanied singing, which was developed in the Catholic church, mainly during the period 800-1000. It takes its name from Pope St. Gregory the Great, who is believed to have brought it to the West based on Eastern models of Byzantine chant.

References

• Canticum Novum, Lessons on Gregorian Chant: Notation, characteristics, rhythm, modes, the psalmody and scores at http://interletras.com/canticum/Eng/index1_Eng.html
• Willi Apel, Gregorian Chant. Bloomington etc.: Indiana University Press, 1973. Fine, digestible first introduction, with an emphasis on musical analysis.
• David Hiley, Western Plainchant. Oxford: Oxford University Press, 1993. Excellent encyclopedic work, good for quick reference and thorough first introduction, much broader in scope than Apel's book, but also at times less in-depth. With the largest bibliography I have ever seen...
• Peter Wagner, Einführung in die Gregorianischen Melodien. Ein Handbuch der Choralwissenschaft. Three thick parts. Leipzig: Breitkopf & Härtel, 1911. After more than hundred years, still the classic study. Obviously dated here & there.