A lot of people realize that, in Harry Turtledove’s alternate Byzantium, the god “Phos” (Light) is both an alternate Ahura Mazda, as well as a way to comment on Byzantine Christianity without commenting. But he’s also referring to the famous Christian hymn “Phos Hilaron”. Like “Inventor Rutili”, it’s a lamplighting hymn. IIRC, Turtledove even quotes the thing in his trilogies, but I don’t know how many people catch it.
Here’s the hymn in Greek, sung to the “ancient” melody (“melos archaion”) by the late Fr. Dositheos, a blind monk from Mount Athos, with a drone backup (Kevin tells me below that the sound’s called an “ison”) from the monastery choir. From analogion.com’s very interesting Byzantine music resource pages.
For those of us who don’t read Greek letters, here’s the transliterated lyrics so you can sing along:
Phos hilaron aghias dóxis, athanátou Patrós, ouraníou aghiou mákaros, Iisoú Hristé, elthontes epí tin ilíou dysin, idóntes phos esperinón, hymnoumen Patéra, Yión, kai ághion Pnevma, Theón, Axion se en pási kairoís hymneisthai, phonés aisíais, Yié Theoú, zoín o didoús, dió o kósmos se doxázei.
The interesting bit is that, although you hear its great solemnity when sung here as a church hymn (and rightly so), after a while the underlying bounciness starts to come through.
Here’s three totally different settings (in English translation) dug up by Chantblog. I don’t know if any of the music is the same as the tune above. Oremus has “O gladsome light” TTTO “Le Cantique de Simeon” by Louis Bourgeois.
Aliens in This World 
Harry Turtledove’s alternate Byzantium …
What is the name of the book or series?
Turtledove actually has actually written several series in his alternate Byzantium world! (And why not? Byzantium’s his academic specialty.) His Byzantium is called “Videssos”, and his Persian Empire is “Makuran”. His map of the empires reverses east and west, and plays some other games as well.
The first series, which starts with The Misplaced Legion, has probably one of the best what if premises ever — what would a classical Roman think of Christian imperial Byzantium? As a first time novelist, Turtledove isn’t able to fully exploit the drama, but it’s still pretty good.
The second series, about Krispos, is based on a real life rags-to-emperor story.
I haven’t read the Time of Troubles series or the villainous heresiarch background book, Bridge of the Separator. (Not that I didn’t want to, but I just never can find them when I’m in the mood. Must hit the library soon.)
Steven Silver has a listing of the Videssos books in reverse order of appearance here: http://www.sfsite.com/~silverag/videssos.html
Turtledove’s an interesting writer, and well worth reading. He has two major problems, IMHO: that he often seems to get caught up in being clever about history instead of letting the story and characters unfold, and that he is a little too eager to sign contracts for long series.
I adore his O’Brien-esque classical Greek merchants series (written under the penname H.N. Turteltaub) because it never falls into either trap. http://www.sfsite.com/~silverag/greece.html
I didn’t realize he’d also written under the penname “Dan Chernenko”. Which explains a lot about a series I glanced at, and thought was very clever about history and Eastern Europe in an alternate fantasy. Man, and I thought he’d started slowing down, when he was just diversifying his business enterprises. *shakes head slowly*
That “backup drone” is called ison, eesohn. In the Latin tradition it’s called organum.
Well, thank you. That’s worth knowing, and ison’s a good word.