Before She Was Helen by Caroline B. Cooney

Clemmie, now living in a retirement village in South Carolina, thinks that she's left her past far behind. When her next door neighbor Dom disappears and a dead body is found in his garage soon afterwards, Clemmie fears her past--that she's worked very hard to conceal--will soon be uncovered. Clemmie, not knowing whom to trust, must go to all lengths to keep the life she is now living intact. Before She Was Helen reminded me of the novels of Hallie Ephron.