What separates The Hilliard Ensemble from other choral groups is that they haven't been afraid to try new things. While most of their work is made up of Medieval and Renaissance music, they've also worked with new contemporary composers. And they are probably most noteworthy for their collaborations with Norwegian sax man Jan Garbarek. Timing being everything, ECM,The Hilliard Ensemble, and Garbarek all scored in 1994 with "Officium"--the first part of a trilogy and an album that found a willing audience at the very peak of the interest in Gregorian chant music.
The group will be retiring, for once and all, with a concert in London on December 20, so its fitting that they should close out their recording career with a Christmas album. But I wouldn't expect an appearance from Garbarek on "Transeamus" (I think he'd be mentioned on the cover).
The album takes its title, "Transeamus", from a traditional Latin Christmas song, "Transeamus usque Bethlehem" (Let us go to Bethlehem), a song as popular in the Netherlands as it is in the Roman Catholic church, for some reason. It is arranged for a four-part choir and usually accompanied by an organ. I can't speak to the full program, though I know "There Is No Rose" is usually part of it.
Anyway, this is entirely out of my depth. But, for those who like this sort of thing, I'm guessing this will be the sort of thing they like.