Elsewhere by Gabrielle Zevin

When 15 year old Liz is killed in a bicycle accident (always wear your helmet!), she goes to Elsewhere. She holds on to the life she used to live , not accepting that a life in Elsewhere can be a full one. Can Liz fully embrace this new, backwards life with new friends and never met family members, or will she keep holding on? This book is a realistic fantasy about friends, family, and how to live your life (or death).

The Elsewhere page on Gabrielle Zevin's official website