The 6:20 Man by David Baldacci

 Travis Devine is a decorated soldier, a former Ranger, who is now an underling at a financial services firm run by a cocky billionaire. One day, Sara, a coworker he had once dated, is found dead in a closet at the company. First it's proclaimed a suicide then the cops decide it was a murder. Soon there is another death, then another. And Travis seems to be the cops' main suspect even though he's innocent.

One day he is called to a meeting with a mystery man named Campbell who knows more about why Travis left the military than Travis is comfortable with. He forces Travis to work for him and find out what is really going on at the financial services firm. 

Travis isn't sure who to trust, so he has to rely on his wits and his military training to keep himself out of jail and one step ahead of the bad guys. It's quite the page-turner. And it appears to be the beginning of a new series by David Baldacci. Good summer read.

PS: The title comes from the fact that he takes the 6:20 train in to work every morning.