This is a workplace-set psychological thriller by Freida McFadden:
The Coworker starts with "a familiar colleague, unknown secrets"—the story focuses on the undercurrents beneath office routines, unpacks deadly workplace dynamics through hidden betrayal and out-of-control survival games, and uses suspenseful pacing to reveal the dangerous truth behind a seemingly ordinary coworker. Suitable for thriller fans and workplace story readers, it lets you experience tense suspense in a relatable daily setting.