Keeper Of The Light by Diane Chamberlain

You're on duty in the emergency room and despite desperate efforts, the victim of a gunshot wound loses her life. She is Annie O'Neill. Your husband's mistress.

Olivia can't watch any longer as her husband grieves the love of his life. Desperate to understand who this perfect woman was, Olivia finds herself stepping into Annie's shoes. Into another family one with an adoring husband and troubled young daughter. A family who make Olivia feel needed, but don't know who she really is. And the façade is cracking with every step she takes.

There's still time to turn back to a husband who loves her second best. Or to keep walking to a new future but in a dead woman's shoes.