Weird and boring.
It's clear that the author had done a lot of research into the castrati to write the book, the thing was just chock full of real historical asides about real people, including an appearance from a young Mozart to talk about his visit with Farinelli and a scene at the front door of a retired Caffarelli's house. The book was historically rich, although I'd question the authenticity of "telling the story" of who the castrati were in Italian culture, nothing was too historically wild in the accuracy in that direction.
Except, of course, pretty much everything else about it is totally wild. Other than our narrator, likable country boy Porporino, the main characters were the Prince (who was a real person), a creepy mad scientist, Feliciano, a fellow castrato and total jerk, and the Duke Don Manuele who is in unrequited love with Feliciano. Using these four, and a few other people, the book seems to alternate between long boring expository segments set in Italian salons, where everyone but Porporino talks forever about whatever the author found in their research, and weird and unsatisfying homoeroticism.
The stars are for the extensive research and the authors' clear affection for castrati, but not much else to recommend the book.