A Victorian-era fantasy children's book, for some reason labelled "Mystery" by the library I got it out from. Not a good label. Suspense, on the other hand, might be.
It opens with a dying old woman, Cassandra, afraid of the phoenix stone that has been the source of her magic. A treacherous student of hers, from her middle years when he was young, had warned her about it, but not enough for her to escape.
Meanwhile, Grisini, her erstwhile student, is in London with a marionette show, and two wards he uses to help with it, Lizzie Rose and Parsefall. They go to entertain at a birthday party of Clara, who is the sole survivor of the cholera in that family. Her mother is so preoccupied with the dead children that they go to the graveyard several times a year -- including the birthday, since it was also her twin's -- and she receives presents from her brothers and sisters. She manages to sneak in tea and have it with the two of them.
When she vanishes soon after, suspicion falls on Grisini, but they are unable to find her at the house. Lizzie Rose learns that twice before Grisini had been suspected when children vanished, though they had never been found, and the children had returned home. Meanwhile, Cassandra decides to reach for Grisini to discover the secret.
It involves the curse on the phoenix stone, mementos of old lovers, a trip by train, Parsefalls stringing up a new puppet, a ransom that does not get paid, the children remembering the story of the Bottle Imp that they perform, a tower that falling to pieces, and a backstory involving a school at a convent. Lots of intriguing twists, and magic, along the way.