Mystery of the Russian

Horrors! ``The largest ruby in the world'' has been purloined from the safe at Wilby Court, home to the Countess Wilby-and site of this book's mise en scene, a snowbound weekend party. In the best Agatha Christie tradition, Smyth introduces the requisite motley crew: an assortment of staff and guests each possessed of motive and opportunity to commit the foul deed. The paper engineering, not the cast, takes center stage here: ``Top Secret'' dossiers on each character can be slid out of the pages, while various flaps and pull-tabs reveal hidden architectural passages, concealed weaponry, and contraband lurking in various valises-each of the six spreads yields enough clues, in fact, to delight Holmes himself. And that master of 221B Baker St. would appreciate the conclusion as well, which features three possible solutions accessed on a revolving wheel. The text and illustrations are somewhat bland, lacking the pop-ups' panache and mysterious mood. Nonetheless, aspiring sleuths will echo the cry, ``Egad, the game's afoot.'' All ages. (Sept.)