Walking Through Stones by K.L. Kerr
Quirky high school sophomore Harley Cattanach is celebrating her birthday alone at her favorite hangout, a decrepit Revolutionary War-era fort that looms above the town cemetery. But Harley soon has unexpected company when the restless spirits of the dead begin calling to her from their graves.The fifteen year-old refuses to believe her ears, dismissing the voices as a prank, until Chester, a painfully shy teenager who frequents the cemetery, confides he hears them too. Harley's world is complicated enough, with first love blooming, the loss of her grandfather, and the constant battling with her mom--she doesn't need to add a bunch of ghosts. But the more she ignores them the louder and more demanding they become. Harley and Chester embark on a series of adventures--passing through gravestones into the past to correct singular events in each spirits life.