As the author of a long serial, Patrick O'Brian has mastered the art of the tease. This installment, probably the penultimate one in his series of Napoleonic sea novels, is a case in point. All the usual elements are here, including a fast-paced plot driven by historic events that cause our heroes, Commodore Jack Aubrey and his friend and ship's surgeon/spy Stephen Maturin, to leap into action. But in telling his tale, O'Brian demonstrates that, 19 books along, he isn't content to merely pen a suspense-soaked sea story. He wants to mess with the minds of his readers. Using all the craftiness of a veteran knuckleball pitcher, he delivers the expected unexpectedly. And then tosses in the truly unexpected. The Hundred Days by Patrick O'Brian (W.W. Norton, $24) The Hundred Days is set in motion by Napoleon's escape from Elba in 1815. The Aubrey-Maturin mission: to disrupt Bonapartist shipbuilders and naval units in the Adriatic and prevent a load of Arab gold from reaching Muslim mercenaries in the Balkans. Said mercenaries are being hired to delay the Russian and Austrian armies from uniting against the newly resurrected French dictator. Our heroes brave battles, storms, and treachery to stop the bad guys. Naturally, the fate of Europe is in their hands. The plot has plenty of water-based battles and is stocked with familiar fare: favorite members of the crew who have sailed through most of the series (the aptly named Preserved Killick among them); essential characters in the shape of anthropomorphized ships (the "happy" Surprise, the "laborious" Pomone); a bit of science and superstition, here in the form of a narwhal tusk; flocks of exotic birds to distract and delight Maturin; and vocabularies filled with colorful jargon ("Put the tray down there, ye thrawn, ill-feckit gaberlunzie.") O'Brian keeps it all fresh by employing offbeat tactics to deliver vital, indeed timber-shivering, news. In the book's opening pages we learn of a shocking turn of events by overhearing the conversation of a couple of strangers. This is a favored O'Brian device; major action often occurs out of view, as it does in real life, making both readers and main characters frustrated equals in their desire to know the truth. It is also an effective way to inject tension into a story, as is killing off characters. O'Brian does both with great coolness: He gives us an unexpected suicide and, late in the tale, offs a longstanding member of the crew. In The Hundred Days, Maturin, in particular, must deal with traumatic events (which won't be revealed here), but O'Brian decides to let us imagine their real consequence. So, while Aubrey and Maturin's external adventures follow a familiar pattern, their inner lives—which is what lifts this series from Horatio Hornblower to Jane Austen status—are a bit of a question mark. Napoleon may be headed toward his Waterloo, but what about our friends? How will they cope with pain and loss? Hell, how will they cope with peace? That's the emotional cliffhanger that will keep fans in a state of eager unease until the next book (working title, True Blue). Word is that it will wrap things up before the age of steam takes the wind out of Aubrey and Maturin's sails, though O'Brian is said to be ambivalent about ending his series. It would be sad to reach the end of this masterful 20-volume voyage, but O'Brian knows his art: It would be sadder if he left us up in the air.