Glisteningly detailed period adventure--by a pseudonymous pair of historians. England is at war with France, and Bonaparte has massed 50,000 troops along the Channel coast, clearly envisioning invasion. So Captain John Valcourt Justice is asked to go undercover to France and stop Napoleon's fleet at Boulogne--a mission which was in the hands of one Matthew Fielding, who has now been arrested and interned (along with other Englishfolk) at Verdun. Justice enters France by dinghy, pretending his fishing smack has capsized; he's captured, questioned at length, then sent to Verdun--a high-fashion prison-camp/resort--by way of Boulogne (where he sees the French fleet). And all the while Justice is trying to figure out just how Fielding had planned to foil Napoleon (there are coded clues in a copy of Gulliver's Travels). A round robin of characters leads Justice to an impasse: he must escape to Paris--accompanied by Fielding's former filly Madame Lucienne Lamotte. But finally, when Justice boards a fleet of Boulogne brigs which Fielding had hired and loaded with huge casks of wine, he discovers--rather too late--that Fielding had planned to block the shallow harbor with sunk ships. . . . Superior set decoration (architecture, clothing, furnishings), solid action, so-so characters--a naval-action series debut with special appeal to readers who prize pictorial and historical values.