Elsewhere by Gabrielle Zevin

So what does happen after you die? In this book, people and their pets go to Elsewhere, which seems a lot like Earth: You live in a city, drive cars, go to work, visit with friends. But from the day you arrive in Elsewhere, you start aging backwards until you are a baby again and sent back to Earth. In the meantime you meet others who have died, whether stranger, relative, or friend from the past. You'll meet Lizzie, who died at the age of fifteen in a car accident. At first she cannot imagine that she is dead and not dreaming. She meets her grandma who she never met in life. She finds she can speak canine, and so is helpful to newly arrived pets. You'll follow her on her journey from disbelief, to adjustment, to fulfillment, to love, and back to starting life over again.