And his most recent book is The Math Instinct: Why You're a Mathematical Genius Along With Lobsters, Birds, Cats and Dogs.

NPR: The Math Behind Pellicano's Code

The case has gripped the British public for more than a year and a half, since Williams, an MI6 agent known for his mathematical genius and codebreaking talent, was found dead at age 31 in August 2010.

CNN: STORY HIGHLIGHTS