A fifth outing for cool, reserved, socially inept Sigrid Harald of the NYPD, who now has replaced the vivacious, gorgeous Francesca Leeds as artist Oscar Nauman's lover. Lady Francesca wants Oscar to agree to a retrospective of his work to benefit the beleaguered Erich Bruel House, which has a modest and mostly unvisited collection of turn-of-the-century art. Before plans are finalized, however, the newest (and much disliked) trustee, art historian Roger Shambley, is murdered in the dead of night by a blow from a Victorian cane. Whodunit possibilities include Pascal--the young, beautiful, and retarded janitor--and Richard, grandson of octogenarian Munson, senior trustee and art-gallery owner. The boys moved the body in order to hide their ""friendship."" Then there are also: a shipping magnate who buys stolen art; a docent whom the board passed over in favor of Shambley; and the House director and a gallery owner who takes kickbacks for falsified insurance appraisals--all grist for Shambley's blackmailing mill. In unconvincing fashion, Sigrid and her cohorts' stodgy detective work eventually nails the murderer--and everyone else is free to go about Christmas shopping. Flat prose, and Sigrid, even when she unbends, is still dull. Of marginal interest.