The Templar Legacy (Cotton Malone #1) by Steve Berry

Ex-agent Cotton Malone used to work for Stephanie Nelle in the US Justice Department. Now she seeks his help again to crack a series of puzzles that have confounded experts for centuries - and could lead to the legendary lost treasure of the Knights.

But Malone soon realises someone else is on he trail - someone prepared to commit the ultimate crime in pursuit of the ultimate prize. Malone and Nelle find themselves in a heart-stopping race through the villages, castles and cloisters of Europe in pursuit of a secret that, in the wrong hands, could bring the world to its knees.