The blokes at MI6, wondering where unassuming Prague dealer Pavel Pesanek got the Rembrandt he sold for $30 million, have dispatched Czech-born agent Jan Capek to ask him. But it's too late for Capek to ask the late Pesanek, so his masters have to go back to the answer they already know: He got the painting from the vanished collection of that distinguished patron of the arts, executed Romanian President Nicolae Ceauescu. Who fed Pesanek the canvas, and where is the rest of the collection? These questions get this cloak-and-dagger intrigue off to a smart start, and Skeggs, staking out his hero between his old nemesis, Major Ludvik Vlasek of the Czech secret police, and the new heavy in town, National Reform Party empire-builder Jaroslav Kupka, knows how to turn a single knowing remark (Vlasek's offhand reference to ``the old days . . . when I was working for the previous management'') into a sly portrait of post-Communist Prague. But Capek's ignorance about Old Masters--he hardly gets within ogling distance of the Ceauescu collection and can't authenticate the paintings even under the very real threat of torture--deprives art-expert Skeggs (The Triumph of Bacchus, 1993, etc.) of his greatest strength. It must be exasperating to have your own hero hamstring you so. Nevertheless, Skeggs shows a nice line in weary Adam Hall- ish disillusionment before the inevitable round of betrayals and fatalities sets in.