Black Opera by Mary Gentle
Conrad Scalese is a writer of librettos for operas in a world where music has immense power. In the Church, the sung mass can bring about actual miracles like healing the sick. Opera is musicodrama, the highest form of music combined with human emotion, and the results of the passion it engenders can be nothing short of magical. In this world of miracles, Conrad is an atheist - he sees the same phenomena, but sees no need to attribute them to a Deity...until his first really successful opera gets the opera-house struck by the lightning bolt of God's disapproval...And Conrad comes to the attention of the Prince's Men, a powerful secret society, who are trying to use the magic of music to their own ends - in this case, an apocalyptic blood sacrifice. Life is about to get interesting for Conrad.