Close My Eyes / Sophie McKenzie.

Enhanced descriptions from Syndetics:

Gone Girl meets Before I Go to Sleep in Sophie McKenzie's Close My Eyes, a riveting psychological thriller about a grieving mother who finds out years after her daughter's death that her child may still be alive

When Geniver Loxley lost her daughter at birth eight years ago, her world stopped... and never fully started again. Mothers with strollers still make her flinch; her love of writing has turned into a half-hearted teaching career; and she and her husband, Art, have slipped into the kind of rut that seems inescapable.

But then a stranger shows up on their doorstep, telling Gen the very thing she's always wanted to hear: that her daughter Beth was not stillborn, but was taken away as a healthy infant and is still out there, somewhere, waiting to be found. It's insane, unbelievable. But why would anyone make that up? A fissure suddenly opens up in Gen's carefully reconstructed life, letting in a flood of unanswerable questions. Where is Beth now? Why is Art so reluctant to get involved? To save his wife from further hurt? Or is it something more sinister? And who can she trust to help her?

Ignoring the warnings of her husband and friends, Gen begins to delve into the dark corners of her past, hopeful she'll find a clue to her daughter's whereabouts. But hope quickly turns into fear and paranoia, as she realizes that finding the answers might open the door to something even worse than not knowing. A truth that could steal everything she holds close - even her own life.