Eighteenth-century opera and alchemy and convoluted plots against royalty and improbable romances! There's a lot of alignment with my interests there, so the only question was whether Burgis could pull it off in terms of the story-telling. Short version: yes.
The newly widowed Baroness Charlotte von Steinbeck has come to stay with her younger sister Sophie at the dazzling Eszterhaza palace, home of Prince Nikolaus Esterhazy who is patron to the talented composer Haydn and host to the famous castrato Carlo Morelli, as well as an odd assortment of alchemists. Prince Nikolaus also happens to be Sophie's lover, and the domestic politics including Sophie's husband and the reclusive princess are only the beginning of the tangle that Charlotte has fallen into.
The story combines a number of honorable and likeable characters, several outright nasty villains, and a larger number of people desperately trying to find a path to survival, whether that path is honorable or not. The prose doesn't stint on the callous violence meted out by people of privillege, or the ruthlessness of those driven either by a cause or by revenge, but neither does it overly dwell on those aspects. The background of musical production and performance in the later 18th century was solidly grounded. The promised alchemy was a bit more on the side of pseudo-Masonic ritual and sorcery than philosophical endeavors in the laboratory, bringing in the fantasy elements that are responsible for the major climax.
Given the genre of the book we are never in doubt that the improbable romance will turn out well. The only question is whether we will believe in that ending. I did, though perhaps I have the advantage of a great deal of reading in the history of the era. Given birth and privilege, one could make some very improbable choices if one were willing to include the necessary sacrifices.
I found Masks and Shadows to be a quick read, and one that passed my "treadmill test" with flying colors. (Since most of my pleasure reading is done on the elliptical at the gym, I evaluate books based on whether they make me lose track of when my session is supposed to end.)