First to Find — Morgan C. Talbot

Death is the hardest puzzle to solve. Margarita Williams escaped death at a young age, but its shadow has followed her all her life. Now, amidst the chaos of a new Australian roommate and mysterious, menacing neighbors, Death has set the puzzlemaker a puzzle of her own: someone is killing her fellow geocachers, one by one. And if she doesn't stop the murderer before he strikes again, Death will finally collect the soul that got away.