The link above is to a pdf from a journal article that talks about this amazing hymn to Mary. The Harp of Glory is from the Ethiopian Orthodox Church, so it is from an Oriental Orthodox church, but much of the ideas and I think a similar long hymn are found in the Eastern Orthodox tradition from Byzantium/Constantinople. Since these churches were schism'd out over a dispute concerning how people with perhaps too much time on their hands wanted to dissect a mystery of faith, the hymn is not as well known in the west as it should be.

To call it a hymn to belies the size of it. It is the size of a small novella, or an epic poem. The writer composed a set of verses for each letter of the alphabet-- I'm assuming the Ethiopian alphabet-- praising Mary. Nearly every line has a foot note showing where in the bible (the massive Ethiopian bible I'm assuming) the justification for the line comes from. It really is a beautiful composition, and I think anyone and everyone should try to get a copy to read a bit of it daily. It is really inspiring.