Early Auden is the “strangest of boys,” a savant who sweeps 13-year-old Jack Baker onto the Kennebec River and into an adventure rife with “pirates, a volcano, a great white whale, a hundred-year-old woman, a lost hero, a hidden cave, a great Appalachian bear, and a timber rattlesnake.” For Early, the number pi tells an ongoing story that somehow relates to the fate of his older brother, Fisher, a soldier reportedly killed in World War II. Pi’s story is interspersed with Jack’s first-person narrative and guides Early, for whom numbers have “color and landscape, texture and voice,” on his quest for his brother. Initially, Jack, who is reeling from the sudden death of his mother, just goes along with his odd friend. But the wilderness journey becomes a way through Jack’s guilt and grief, too, as he begins to recall small things — his mom’s special teacup and funny sayings, his father’s ring — and feel more connected with the world. Clare Vanderpool deftly rows this complex, inventive novel — her most recent since her Newbery-winning “Moon Over Manifest” — to a tender, surprising and wholly satisfying ending. The richly detailed Maine setting allows the reader to experience moss-slick rocks, murmuring water and bear prints as big as pie pans.