Fidelity by Thomas Perry
From Perry's site:
"When we meet a new person in life, we realize we are not only dealing with the person we see, but also with what he's seen and done and heard. I've always tried to reproduce that fact in my books, but this has been a particularly important part of the last few I've written — Nightlife, Silence, and now Fidelity. In a way, Fidelity takes this notion the farthest. In order to succeed, characters must learn to use the knowledge and intuition they have to understand the secrets of a dead man's past."