Best Friends Forever by Jennifer Weiner

When Valeria moves in across the street, nine year old Addie is sure she has found her best friend. As teen-agers, the doughty Addie is often left behind while the attractive Val is courted by the popular crowd. A defining incident at a forbidden party brings their friendship to an end. Fifteen years later, Valerie is once again on Addie's doorstep in the wrong end of a bad situation and needing her best friend's help. It is a story of delightful retribution with amusing visits to the memories of a misguided youth. It is also why many folks avoid their high school reunions.