Hrmmm, hrrmmm, hrrmm...
Well, without going through any spoilers:
The first part details Damiano, a talented musician/ singer in Renaissance Italy trying to get by, especially when the Black Plague hits the land. What makes him interesting is that he's been targeted by Satan because of his teacher-student relationship with Archangel Raphael.
The second half of the series tells the story of Raphael and how he tries to avoid the clutches and traps of old Nick, who really wants the soul/being of God's favorite angel.
Oh, and in case you're wondering, though McAvoy use religious figures in "Damiano", there is not one hint of proselytizing or religious anything in the story so don't worry about it.
Whew! Is that vague enough? (Interesting enough, a google search couldn't get any reviews on the internet.)