The Pact by Jodi Picoult
For eighteen years the Hartes and the Golds have lived next door to each other, sharing everything from Chinese food to chicken pox to carpool duty. Parents and children alike are best friends - so it's no surprise that in high school Chris and Emily's friendship blossoms into something more. They've been soul mates since they were born. When the midnight calls come in from the hospital, no one is prepared for the appalling truth: Emily is dead at seventeen from a gunshot wound to the head as part of an apparent suicide pact. The gun holds a single unspent bullet that Chris tells police he intended for himself, but a local detective has doubts. And the Hartes and Golds, in a single terrifying moment, must face every parent's worst fear: do we ever really know our children at all?