Threshold of Darkness by Christen Ballard:
 
Naomi Potter left behind everything she knew to be with William, but when he vanishes through a mysterious Door, she is forced to face the unknown.

Grief, doubt, and unanswered questions follow in his wake, pulling Naomi into a quiet but escalating struggle between Light and darkness, truth and deception. As she searches for answers, the comforts she once relied on begin to feel uncertain, and the line between safety and surrender grows thin.

What begins as loss becomes a reckoning.

The more Naomi seeks understanding, the more she realizes that neutrality is no longer an option. Forces she does not fully see are at work, pressing in through ordinary moments, familiar relationships, and the desires of the human heart.

Threshold of Darkness is a haunting spiritual thriller that blends mystery, introspection and suspense, exploring themes of faith, grief, and spiritual warfare. Atmospheric and unsettling without being overt, it invites readers to consider what shapes their choices-and what quietly shapes them in return.

Because the most dangerous Doors are the ones we choose to ignore.