It’s not often that I encounter an entertaining, provocative novel that also happens to have a strong mathematical component or even mathematicians as characters. That’s what I found in the case of *PopCo*, written by Scarlett Thomas.

Thomas ably weaves mathematics into her narrative, making it seem quite natural for sophisticated topics such as the Riemann hypothesis, cellular automata, prime factorization, small-world networks, and more to come up in practically everyday conversation.

But the focus is really on code breaking. This is, after all, a mystery story, and the heroine, Alice Butler, has several secret messages to unravel. So, there are inevitable lessons in Caesar shifts, Vigenère ciphers, trapdoor codes, and the famous Enigma machine of World War II. The book even has a table of the frequency of occurrence of letters in English and a list of the first 1,000 prime numbers.

Thomas has a Web site, with several pages devoted to *PopCo*. She provides bonus material in the form of two snippets that were cut from the final version of *PopCo*, one dealing with Georg Cantor (1845–1918) and the sizes of infinity and the other about Alan Turing (1912–1954) and computability (see http://www.bookgirl.org/index.php).

Perhaps it shouldn’t be surprising that cryptography, with its trappings of intrigue and suspense, can play an important role in a novel. The massive code-breaking effort at Bletchley Park in England during World War II is featured prominently in two relatively recent novels: *Enigma* by Robert Harris and *Cryptonomicon* by Neal Stephenson. I particularly enjoyed *Enigma*, partly for its seemingly realistic portrayal of the effort to break the German Enigma machine codes.

Another intriguing mystery that also includes secret messages and cryptography—and the mathematician John Wallis (1616–1703) as a character—is *An Instance of the Fingerpost* by Iain Pears, which looks at the same crime from four different viewpoints.

*Smilla’s Sense of Snow* by Peter Høeg doesn’t involve cryptography, but it was an unexpected pleasure in its own way when I read the book. The heroine of this thriller is a scientist, and the book includes a number of math-inspired passages, including one on David Hilbert’s hotel with an infinity of rooms, all filled, always having room for one more guest.

For additional examples of fiction laced with mathematics, see the listing compiled by mathematician Alex Kasman of the College of Charleston at http://math.cofc.edu/kasman/MATHFICT/.