Marly's Ghost — David Levithan

When Ben's girlfriend, Marly, dies, he feels his life is over. What could possibly matter now when Marly is gone? So when Valentine's Day approaches, it makes sense that this day that was once so meaningful to Ben leaves him feeling bitter and hollow. But then Marly shows up--or at least her ghost does--along with three others spirits. Now Ben must take a painful journey through Valentine's Days past, present, and future, and what he discovers will change him forever.