Agent of the Unknown was originally published in Startling Stories in 1951. This was an abridged form and the full novel was issued as part of an Ace double book five years later with Philip K. Dick's The World Jones Made. As this novel was written in the midst of her most prolific writing period, it seems the best place to end the series.
Agent begins with Don Haing waking up on the beach. He's hung-over again and desperately in need of a drink. Don's a beach bum, who makes his money finding objects tourists have left in the sand and selling them to anyone who will buy. But this is no regular beach. He's stuck on Fyon, an artificial planetoid created as a resort for the wealthy. Needless to say, the novel takes place in the distant future.
But Don's luck has changed: he's found the most wonderful miniature doll buried in the sand on the beach where he sleeps every night. No bigger than the length of his hand, the doll resemblesthe perfect form of a nude woman. And she's weeping, or at least seems to be weeping. Sure this doll will bring him some cash, Don takes it to his friend Kunitz, who lives in a small house by the beach. Kunitz informs him the doll is priceless; the only other one like it exists in a museum on Earth. Both were made by Vulcan, a master craftsman who lives on the edge of galaxy.
Although Don wants to sell the doll, he discovers he can't. A tourist who tries to take it off him gets an electrical shock. Soon, Don notices that his alcohol craving has disappeared. He takes a job as a short-order cook at a local bar and shows the doll to the bar owner. Once the bar owner sees the doll, he too becomes fascinated with it. It's not long before everyone Don meets wants the doll. Eventually, The SSP, the only organized force in the galaxy, starts sending out agents who are determined to get the doll away from Don at any cost.
St. Clair shows her classics background in this novel. The craftsman who makes the doll is known as Vulcan (Roman god of metal work). One of the villains in the story has the name Mulciber (another title for Vulcan). The prison planetoid where the SSP takes their victims is known as Phelegthon, one of the five underworld rivers in Greek mythology. The final appearance of Vulcan at the novel's conclusion resembles something out of Bulfinch.
One again, the future looks bleak. Most of the people can't read, using "isotypes". After a plague, known as pyrexia, has killed the bulk of humanity, the SSP (Special Serum Purveyance) remains the only government in the galaxy. The SSP controls all scientific research with an iron fist and rigidly hunts down any mutants. Psychotropic drugs keep the population in line. But change is coming.
A solid book with a powerful conclusion.