After losing her job as a North Dakota sheriff’s investigator following a tumultuous serial killer investigation in “Paradise Valley,” Cassie Dewell resurfaces as a struggling Montana private detective in C.J. Box’s new mystery “The Bitterroots” (Minotaur Books, $27.99).

She doesn’t want to work for defense lawyers. But, as the single mother of a teenage boy, the former cop can’t afford to say no when one asks for her help.

Dewell is repulsed that the lawyer’s client — Blake Kleinsasser, the oldest son in a clan that owns the vast Iron Cross Ranch and just about everything else in town, who’s back in Montana after working as a New York stockbroker — is accused of raping his 15-year-old niece.

But the lawyer tells her that if what she finds backs up that he’s guilty, he’ll talk Kleinsasser into a plea deal.

Dewell ventures into the mountains of western Montana to investigate and is met with hostility and intimidation by nearly everyone in town, including his father and brothers, who despise him for turning his back on the family’s ranching heritage.

If the rape case against him is so solid, Dewell wonders why everyone is going to so much trouble to keep her from doing her job.

Kleinsasser is beaten in prison and left in a coma. By then, Dewell discovers that the rot in the family runs deeper than she could have imagined.

Box is best known for his mystery series featuring Wyoming game warden Joe Pickett. In this offering in his new series, his writing is vivid, his characters are well developed, the tension runs high, and the plot unfolds at a rapid pace.

DARIEN APPEARANCE THURSDAY: C.J. Box has a book-signing from 7 p.m. to 9 p.m. Thursday at Frugal Muse Books, 7511 Lemont Rd., Darien.