The Negotiator (The Guardians #1)
Kay David

Beck Winters is the Negotiator.
Despite everything he's seen and done, Beck's still sane, but only because he never allows himself to get close to anyone. Then during a hostage incident, he hears a voice over the phone line, Jennifer Barclay's voice.
Jennifer's in the wrong place at the wrong time. Her plan has always been to avoid excitement. But that was before she was taken hostage...before the incident ended badly...and before she met the Negotiator.