A Conspiracy of Paper is really more of a genre novel, a detective novel, and it's often difficult to ascertain a "theme" in a novel like this. However, if I had to choose something that masquerades as a theme it would be that of "identity." In 18th century London, Jews were considered inferior citizens. There were many rumors of their imminent expulsion from the country, and they were easy targets for everything from extortion to scapegoats. Liss's protagonist Benjamin Weaver runs away from home at an early age and changes his name. Some may say that Weaver is abandoning his heritage, his identity as a Jew, but that clashes with his later use of "The Lion of Judah" as his fighting name. Still, Weaver neither really acknowledges nor practices the rituals associated with his Jewish heritage. When his father Samuel Lienzo is killed, Weaver begins to investigate what increasingly looks less like an accident and more like murder. During the course of his investigation, he must make contact with his uncle Miguel Lienzo, who can give Weaver the most insight about his father. He also develops a love interest in Miriam, his Uncle Miguel's daughter-in-law, and has dealings with Nathan Adelman, a financier who has ties to the company Weaver is investigating. In addition, Weaver has several encounters with a boyhood friend, Abraham Mendes. All of these characters are Jewish and force Weaver to re-examine his own Jewishness. As his Uncle Miguel reintroduces Weaver to the rituals associated with his heritage, Weaver begins to find a sense of identity and security that society does not afford him.
The novel also potentially examines social Semitic stereotypes, positioning Jews both as 'stock jobbers,' or brokers, a socially inferior position, and, in the case of Adelman, as financier, a financially superior position, yet also one that plays into the stereotype of the rich-but-cheap Jew.