While looking through my personal Christmas music collection I noticed something. I own a lot of recordings by Andrew Parrott and his Taverner Consort, Choir and Players. Parrott specializes in authentic, or "period" performances. When possible he returns to the original manuscripts (or as close as he can come) and performs in the styles when the compositions were originally heard. He has also published major articles on Monteverdi, Purcell and Bach.
Parrott's recording of Handel's Messiah is one of my favorites, with delicate vocals from tenor James Bowman and alto Margaret Cable among others. But it's his series of carol albums that I enjoy the most during the Christmas season. They are titled "The Carol Album," "The Carol Album 2" and "The Promise of Ages." The Promise of Ages differs a bit from the first tow in that Parrott includes modern carols. But they blend well with the older works on the disc.
Enjoy Parrott's recording of the Coventry Carol: