After waging two separate quests (The Lost Hero, 2010; The Son of Neptune, 2011), the Greek and Roman demigods of Riordan''s Heroes of Olympus quintet join forces. With his now-trademark zero-to-60 acceleration, the author engineers a ghostly possession to set Greeks and Romans at odds and initiates the Prophecy of the Seven, hurtling Annabeth, Percy, Piper, Leo, Hazel, Frank and Jason into a pell-mell flight on the magical trireme Argo II. They seek the titular Mark of Athena, which they hope will provide the key to defeating the vengeful Earth mother, Gaea, or at least some of her giant offspring. As the trireme crosses the country, the pace drags while the demigods sort out relationships and work to figure out both cryptic prophecy and nightmare visions. With sweethearts Annabeth and Percy once again united, much of the tension that powered earlier books is gone. Once the Argo II leaves the United States, though, the pace picks up, and the comically instructive set pieces Riordan''s so good at emerge. A Luddite god rails against what he calls the "b-book," which displaced the far superior scroll technology; Annabeth gets a crash course in the cult of Mithros far below the streets of Rome. Here, Riordan''s infectious love for his subject matter really comes through, even as he takes some real risks with his characters. A literal cliffhanger leaves eager readers hanging; next stop: Greece-and Tartarus. (glossary) (Fantasy. 10-14)-Kirkus