The usual sequel takes characters from an established book or series and then moves them on toward new adventures. But “The Likeness,” Tana French’s second novel, has a much tighter bond with its predecessor. Her first book was the Edgar-winning “In the Woods,” in which a police detective named Cassie Maddox took on an undercover role. Cassie became a college student named Alexandra Madison in the course of a long investigation that left Cassie with stab wounds and nightmares. But the fictitious Lexie Madison is long gone by the time “The Likeness” begins.  Except she’s not. Cassie is pulled off her current beat, domestic violence, and sent back to work on a murder investigation. The reasons for the switch are grimly self-evident once Cassie gets a look at the victim. Not for nothing is the new case labeled Operation Mirror: there is a dead Lexie Madison, and she looks like the spitting image of Cassie. But Lexie was supposed to be a figment of Cassie’s imagination.  At the behest of Frank Mackey, Cassie’s acerbic, wisecracking boss and an irrepressible Irish charmer, Cassie is sweet-talked into trying the unthinkable. The dead young woman was also a student, and she lived with four others in a bizarrely intimate state of camaraderie. The five of them, two women and three men, shared a big old house that one of the men had inherited. Whitethorn House is outside Dublin and close to Glenskehy, which was once a feudal village to house the mansion’s servants. Today’s Whitethorn residents are about as popular with the locals as the former, aristocratic owners used to be.  Near the house is a cottage. And inside the cottage is Cassie’s dead doppelgänger. But when the police find the body, presumably before anybody else has, Frank hatches his plan. The roommates will be told that Lexie has survived the attack and will be returning home soon. Cassie/Lexie will go back to living with them. And the bandage on her stab wound will hide a microphone that enables Frank to eavesdrop to his heart’s content.  “The Likeness” teases considerable suspense from this tricky arrangement. Before Cassie goes anywhere, she must be drilled in every known aspect of Lexie’s behavior, even though some identifying traits will be impossible for her to learn. She is taught about the things Lexie liked, for instance; but no one can tell her what Lexie didn’t like or wouldn’t do. Cassie’s masquerade is so perilous that at one point she nearly blows her cover by eating onions at a communal meal. The house’s four other residents know that Lexie would deliberately pick the onions out of her food.  The others live such a strange, hermetic existence that small predilections count for a lot. There is such a spooky atmosphere about this household that it’s hard for Cassie to fathom at first. The four  chilly, paternal Daniel; handsome and peevish Rafe; the oddball Abby, with a strange fondness for antique dolls; and easily rattled Justin  have made a peculiar enough pact to give “The Likeness” some resemblance to Donna Tartt’s “Secret History.” They guard their privacy so closely that it is quite mysterious, even by the standards of college students with active fantasy lives.  All four of them, apparently with the cooperation of Lexie, have chosen to live in some realm outside the modern world and to make Whitethorn House a monument to their eccentricities. But what really held them together? What went on within their snobbishly tight little clique?  Scrappy, irreverent Cassie would seem to be a poor match for this precious little crew. But she finds herself unexpectedly dazzled by them and drawn in by the warmth of their affection. And the friendships of Whitethorn House are so close that the place sometimes seems like home to a cabal. But at least one housemate must surely be troubled. Imagine the surprise, even consternation, when the supposedly dead Lexie comes back home.  As “The Likeness” watches Cassie infiltrate this world of secrets, it also follows her combative but highly entertaining relationship with Frank. He too is a skilled faker, since he must pretend not to know Lexie once the police become involved with the residents. And he may not know her well anyhow.  As Cassie is drawn more closely into Lexie’s life, she begins letting her microphone malfunction at crucial moments. So if she is ever in great danger, the policemen who spy on her as she spies on Whitethorn House may not know she needs rescuing.  The conventional approach to this suspenseful setup would have been to make one Whitethorn resident or Glenskehy villager a secretly murderous figure and save the details for a frenzied denouement. But “The Likeness” intends a much longer and more leisurely look at the householders’ domestic arrangements, their inner lives and the yearnings they trigger in Cassie, who has no real home of her own.  Her ties to a co-worker who is in love with her are badly strained by her undercover arrangement too. Nothing in her real life has prepared her for the quaint, anachronistic and sinister daydream into which she has stumbled.  Ms. French resists genre conventions defiantly enough to have written a long, rambling book, one that is more interested in character revelations than in “Aha!” moments about the plot. She could have achieved the same effects much more succinctly in a more tightly edited version of this same story. But Cassie herself remains a strong enough character to sustain interest, even if many of her observations about Whitethorn have a vague, hazy quality. All she needs is a sparring match with Frank, and Cassie quickly returns to the land of the living  and to the subtle demands of her perilous, suspenseful masquerade.
In this unusual sequel, the police Detective Cassie Maddox goes undercover as a dead woman after her spitting image is murdered.