""Perhaps every tragedy has its roots in some little act,"" considers Margot Humbert, the heroine of Gilbert's (The Wedding Guest) twisty and suspenseful gothic tale with a mystery in the middle of its marriage plot. In 1923, many in the rural English village of Ashlaw have not yet recovered from the ravages of WWI. Linden Grey and her mother, a war widow, suffer ""straitened circumstances"" and are eager to develop their friendship with the relatively prosperous Humberts, whose soon-to-be marriageable son, Alex, is Linden's age. Alex quickly becomes ""spellbound"" by the enchanting Linden, but his sister, Margot, begins to realize that Linden's cool perfection and beauty mask an empty soul and a ""squalid self-interest."" When a simple-minded village girl falls to her death down an abandoned mineshaft after being accused of stealing a strand of imitation pearls, Margot is sure that Linden is involved and lying about it. The girl's death triggers a series of events that devastate Margot's family and cause Alex to leave for Africa, prompting Linden to seek an attachment to another promising and propertied young man. Linden's new prospect happens to be Margot's devoted but undeclared lover, whom Linden successfully manipulates away from her rival. Gilbert's characters are richly drawn: cold Linden; her na ve suitors; Margot, who wonders if Linden is an ""instrument of evil like the witches of old, through no choice of their own""; and a classically gothic, slightly mad serving woman who wants to destroy the haughty Linden and who holds the key to the mystery of the missing pearls in this taut, suspenseful tale. (Aug.) FYI: Gilbert's most recent novel, The Treachery of Time, won Britain's Catherine Cookson Award for fiction.