Lock Yourself In. Nightlock Sins, #12

"Every wall is a love letter to the space it protects. Every lock is a confession."Carys Flint doesn't miss walls. As a structural assessor, she reads buildings the way others read books. When she is hired to evaluate 1440 Vesper Drive, a 1932 Art Deco masterpiece, her instincts immediately scream that something is wrong. The walls are inches thicker than the blueprints claim, hidden currents of air flow through the dining room, and at night, she can hear someone breathing behind the plaster.
She isn't alone in the house. Hidden within the very architecture of the building is Atticus March, a disgraced architect who has been living inside a secret network of passages for eleven months. Atticus is hiding to gather evidence against a deadly underground smuggling and corruption ring operating beneath the house. But his isolation has bred a dark, all-consuming fixation. He manipulated the hiring process to bring Carys to his home, having spent two years secretly designing and building a hidden room perfectly tailored to her every preference.
When the dangerous men from the smuggling ring breach the property, Carys is forced to rely on the voice in the walls. "Lock yourself in, " he tells her. And she does. As the conspiracy deepens-tying back to the darkest parts of Carys's own family history-she must navigate a labyrinth of structural lies and deadly secrets. But the greatest danger might not be the men trying to break in. It might be the man who built the walls around her, whose unnerving, intense devotion blurs the fine line between a sanctuary and a cage.