When hotshot young investment banker Edward Wozny is called to the home of an important and mysterious client, the last thing he expects is to be ordered to uncrate and organize a library of rare books. Edward's indignation turns to curiosity when he learns that among the volumes there may be hidden a unique medieval codex, a priceless treasure kept sealed away for many years and for many reasons.

Enlisting the help of Margaret Napier, a passionate and brilliant medieval scholar, Edward learns the strange history of the codex's author, Gervase of Langford, as well as the dark, intricate tale that lies within the missing medieval text. As Edward's obsession with the codex deepens, friends introduce him to MOMUS, an addictive computer game set in a fantasy world that, perplexingly, begins to parallel the legend of the codex. Yet MOMUS confounds more than it clarifies, and it becomes evident that someone is trying to prevent Edward and Margaret from ever finding the elusive codex. As they race against an unknown enemy, the two begin to uncover secrets that the codex's powerful owner will do anything to keep hidden.

An accomplished, powerful literary thriller, Codex explores dark mysteries of both the medieval era and the present, keeping readers guessing right up until its astonishing conclusion.