Hi, lagin,
I didn't know this thread existed until just now... (sorry)
An anonymous treatise, the Musica enchiriadis (Music Handbook), together with a dialogue associated with it, the Scolica enchiridis (Excerpts from Handbooks), are the earliest texts clearly describing and illustrating music composed for more than one vocal part. They were written in the late nineteenth century and are most known for their descriptions of organum, though they also include the oldest notated forms of chant melodies, more developed theory of the modes than found in other writings of the time, information about performance practice, and more. An interesting aspect of these texts is that they clearly refer to a practice already in use, not a new one being proposed.
Hope that helps,
m1469
ps- just for the record, and because I deeply respect my friend, "whynot" is the one who helped you with your random Medieval piece, not me (though I would have been more than happy to if I could have).