Ceremony in Death (In Death, #5) by Nora Roberts

Ceremony in Death follows Lieutenant Eve Dallas as she investigates the suspicious death of a fellow police officer who appears to have been involved in a dangerous underground cult. The case leads Eve into a disturbing world of rituals, corruption, drugs, and occult practices, while powerful people attempt to keep the truth hidden. As Eve digs deeper, she discovers that the conspiracy reaches into the police department itself, forcing her to question whom she can trust. Meanwhile, her relationship with billionaire Roarke continues to develop as he supports her through the increasingly dangerous investigation. Combining murder mystery, futuristic technology, romance, and psychological suspense, the novel pushes Eve into one of her darker and more challenging cases.