Book The Next Thing I Knew by John Corwin
When Lucy Morgan drops dead along with everyone else on Earth she refuses to take death lying down even if, technically, her corpse is.She drags her ghostly social life back from the grave and enlists her friends to figure out the rules of the afterlife. More importantly, they want to discover who or what killed everyone and why the heck anyone would do such a mean thing.But what they discover changes everything. And if they can't figure out how to put their newfound ghostly powers
to work, humanity will be extinct for good.