Subtitled ""A Novel of the Civil War,"" the late van Wyck Mason's farewell novel is as blood-and-thunderous as ever--an enjoyably rickety plot riveted together with period detail and delivered in a tone of lofty care for fact. Lts. David Dexter and Ira Thatcher are returning from embassy duty in Europe and meet in Bermuda, where they report on the state of naval preparedness in England and France, especially regarding ironclad ships. It seems that the Confederate Navy is converting the Merrimac into a submersible armored giant, a ship that will be invincible against Abe Lincoln's ""paper blockade."" In response, the U.S. Navy is building the Monitor, and even an unknowledgeable reader will know that these two ironclad underwater battleships are fated to meet in ringing combat. So, while David and Ira get involved in plots that will lead them to that naval clash (both are loyal to the Union but have Southern ties), non-military subplots weave in and out: Kitty Hamilton, a musical quadroon from Bermuda, becomes a high-class white slave at Adah Markham's Elite Tonsorial Salon in N.Y. before General Carl Brebner buys her release (she's a ringer for his late daughter) and takes her to St. Louis as his housekeeper; Mme. Arlette Louise d'Aubrey, a French spy passing as a dress designer, carries on a love affair with English war correspondent Lionel Humphrey, who attends every vital naval battle. Finally, just about everybody winds up at sea: David ""joins"" the Confederate Navy as a spy, boards the Merrimac, and serves in battle against best friend Ira on the Monitor. (The big showdown is well-done, claustrophobic, ripping and singing with steel shards as the two ironclads cannonade each other.) But Ira dies in a later battle, and David, disabled and pensioned, falls for quadroon Kitty. Likable, rugged, old-fashioned historical-novelizing, best in its vivid picture of life aboard the ironclads--a vigorous goodbye from a popular, effective craftsman.