THE LINE BETWEEN cynical repetition and elegant variation is as fine in adventure stories as it is in - well, as it is in that line the members of Spinal Tap sapiently discovered between clever and stupid. Rick Riordan, after the well-earned triumph of his two Percy Jackson series, tales of an all-American boy who discovers that he is the son of a Greek god, now embarks, with "Magnus Chase and the Gods of Asgard: The Sword of Summer," on an entirely new series, this one about a not-quite-all-American boy who discovers he is the son of... a Norse god. The boys are certainly different - Magnus, our new hero, a Kurt Cobain look-alike, starts out homeless and a petty thief in Boston, where Percy was at boarding school. The "Percy Jackson and the Olympians" and "Heroes of Olympus" series, to give their official names, certainly earned all their successes. Riordan, a teacher before he was a writer, taught a generation of American children not just the names but also much of the substance, the emotional architecture, of Greek mythology. The classical panoply of gods and demigods - satyrs and centaurs and even one girl who, quite properly and unpuritanically, was sired by Zeus and born to a television starlet - were all made new. If one price paid to keep kids involved was to give the characters overly neat, X-Men-style powers, so cheating the original myths of their strange violence - violence and combat not being quite the same thing - this seemed a small price paid for clarifying, as nimbly as Riordan did, the differences, say, between the Greek and Roman mythological pantheons. In the new book, Norse myths are given the same carefully detailed exposition. The magic craft of dwarves, the mission of the Valkyries, the surprisingly variable sizes of giants, even the division between the godlike orders of Aesir and Vanir - all are made beautifully clear. Magnus, our hero, is the son of Frey, the god of plenty, where Percy was the son of Poseidon, and so his sword is the summer sword. His quest is more episodically comic in tone than the Percy series often was (we learn that Magnus has watched William Goldman's "The Princess Bride" 26 times, and some of that tale's facetious humor has slipped into this one). It pits him against giant eagles and sea serpents, with a diverse coalition that includes a young Muslim Valkyrie, in a desperate struggle to put off the day of Ragnarok, the apocalyptic battle the forces of good are doomed to lose. Some of the comedy is very charming. Valhalla is a convention hotel, complete with alarming announcement boards ("Buffet Lunch to the Death! - Dining Hall, 12 p.m."), and Aegir the mead maker is always "talking about microbrews." But some of it feels a bit dutifully antic: The evil prankster god, Loki, is introduced wearing a Red Sox jersey and eating a Pop-Tart. (One of the book's more winning conceits is that Boston is really, truly the hub of the world; the entrance to the world tree Ygdrasil is in the Boston Commons statue dedicated to "Make Way for Ducklings.") The action, at moments made confusing by the device of characters dying and being born again, unfolds predictably but convincingly through various betrayals and plots to the inevitable triumphant alliance of eccentrics. If at times the tone is unfortunately like that of a "Transformers" movie, all wiseguy plucky kid versus Orson Welles-basso-voiced enemy demiurges, at other moments the secondary characters seem convincingly imagined both as old myth types and as comic sidekicks. One wonders, though, if Riordan has caught enough of the special quiddity that separates Norse mythology from other kinds, or even winged it adequately. Greek myths have the distinctive quality of capturing human passions as personal types; the essential gift of the Greek myths is to personify forces as characters. Riordan very skillfully modernized that in the Percy books. Annabeth, daughter of Athena, was a more than plausible recreation of the Greek ideal of stern yet supple feminine wisdom. So too, in the Percy Jackson books, the Greek truth that the best teachers are the wildest men spoke to us freshly - and those books taught something, too, about the classical notion of the antic nature of desire. The essence of Norse mythology lies in its fatalism, its sense that the eternal balance of good and evil may well be stacked against the good and yet the good still has a duty to go on, delaying, at least, the day of Ragnarok. The moody gift of Norse myths is to offer us tragic necessity as nobility of purpose. This was the tone Tolkien's books achieved; his elves fought for Middle-earth even as they were doomed to leave it. Riordan does try to differentiate his tragic Norse myth types from his more canny Greek ones. He has a character observe at one point, apropos of the way Ratatosk the evil squirrel seeks the destruction of his own home in Ygdrasil, that "people have destructive impulses. Some of us want to see the world in ruins just for the fun of it... even if we're ruined along with it," and he seems to make a point of having his Norse gods be considerably dumber than his Greek ones were. But the heroic simplicity, even the rusticity, of the Norse myths seem not very well distinguished from the allegorical elegance of the Greek ones. The rules of American adolescent entertainments are as rigid as any known to other myth systems - there must be that anachronistic humor, action scenes every 30 pages, the triumph of the adolescent boy and girl over the corrupt elders - and though in hands as expert as Riordan's they make for admirably paced entertainments, they do crowd out the possibilities of nobility or wonder. Myths provide marvels. They also make mysteries. Riordan here, as before, does a terrific job of turning old marvels into the mechanisms of contemporary adventures. But his writing still lacks any shimmer of the sublime - an effect that, in the best children's books, need not be self-consciously portentous or solemn: The moments in Mary Poppins books when mythical creatures intrude on London are among the most inexplicably magical in all of Pamela Travers's works. The marvels of myth Riordan recreates here as before; the mystery of myth remains unactualized in his work or, sadder and more likely, unasked for by his time. ADAM GOPNIK is a staff writer at The New Yorker and the author of two fantasies for children, "The King in the Window" and "The Steps Across the Water."

Ever since his moms murder two years ago, Magnus Chase has lived on the streets of Boston, watched over by Blitz and Hearth, a pair of quirky homeless guys. On the day he turns sixteen, however, his estranged uncle Randolph freaks him out with the news that his long-absent father was a Norse god. Then Magnus retrieves a magical sword from the bottom of the Charles River. And then he falls in deadly combat with Surt, the fire giant who shows up to claim the sword. Being a smart-alecky Riordan hero, at least Magnus gets to joke about his own death, especially once a part-time Valkyrie named Sam al-Abbas scoops him up and carries him off to Hotel Valhalla. Although the first third of the book is slow going, stuffed as it is with asides, flashbacks, dream encounters, and digressions (there are 72 chapters in all), once Riordan hits his stride it becomes a whirlwind of myth, action, and wry sarcasm, perfect for readers hungry for a new hit of that Percy Jacksontype magic. The fact that Magnus has a gray-eyed cousin named Annabeth? Just a little something for fans to mull over while waiting for the projected sequel. anita l. burkam (c) Copyright 2016. The Horn Book, Inc., a wholly owned subsidiary of Media Source, Inc. No redistribution permitted.