The elegant mysteries of the British author Peter Dickinson usually come equipped with a gimmick. In _B_The Yellow Room Conspiracy _b_(261 pages. Mysterious Press. $18.95), he marries the travails of a family uncannily resembling the Mitfords to a scandal that echoes the Profumo affair. Toss in some house envy right out of "Howards End," and you've got a plot that sounds unbearably borrowed. It is a measure of Dickinson's artistry that by the end of the first chapter you're no longer bothered by this.
Two aging lovers, Lucy and Paul, narrate the story in alternating chapters. Each begins with the assumption that the other one murdered Gerry Grantworth years before. Grantworth was Lucy's old flame and Paul's old pal, but as each adds bits to the tale, the murder victim emerges as a stranger to them both. As they unravel his character, they edge ever closer to the identity of his murderer.
As always, Dickinson's chief interest is not crime but character. Lucy Vereker and her four brainy, flaky, horsy sisters rank with his most memorable creations. The best of these threadbare aristocrats is Lucy, a woman betrayed by her passions but preserved from tragedy only by her luminous, invincible wit. Describing the proper requirements in a mate, she declares that "it doesn't much matter not having the same kind of good taste . . . But you've really got to have the same kind of bad taste. If one of you likes damask lampshades with gold tassles, then you've both got to." Not even Sherlock Holmes ever gave advice that good.