Extreme Denial by David Morrell

Extreme Denial - Steve Decker is an CIA agent.  He is in Rome to oversee an operation by another agent, Bryan McKittrick.  The operation goes bad because of McKittrick's ineptitude.  However, because of McKittrick's father intervention, Decker is blamed.  Decker, already considering his life needs change, decides to retire from the agency.   Now, living in Santa Fe and working as a real estate agent, he meets and falls in love with Beth Dwyer.  As they are sleeping one evening, a group of men attack his home, Beth is shot, but Decker is able to kill all the men.  Later, when Decker returns Beth home, her home is destroyed in an explosion and Beth has disappeared.