Running Steel

In the sixth book in Stuart Field's thriller series, John Steel is drawn into a deadly game of cat and mouse as he hunts for a methodical killer known as The Sentinel.
Helping an old friend in the CIA to hunt down the serial killer, Steel soon realizes that the culprit is sending them postcards with clues to his next victim. Digging deeper into the lives of the previous victims, it seems that the only thing that is constant is that all cards are addressed to him, Detective John Steel.
Racing against time, can he stop the elusive killer on his tracks, or will he end up as the last name on The Sentinel's list?