If you're a diehard must-be-read-in-chronological-order person:

The Enemy (the prequel);

Killing Floor;

Die Trying;

Tripwire;

Running Blind (US title)/The Visitor (UK title);

Echo Burning;

Without Fail;

Persuader;

One Shot;

The Hard Way;

Bad Luck and Trouble;

Nothing to Lose;

Gone Tomorrow;

61 Hours;

Worth Dying For.