“Big Friends’’ By Linda Sarah, illustrated by Benji Davies, Henry Holt, $16.99, ages 4-8

Is three really a magic number? Birt, one of three boys in Linda Sarah’s and Benji Davis’s moody, poetic “Big Friends,’’ sure doesn’t think so. Life with his best friend Etho is just fine until another boy, a “tiny’’ one named Shu, shows up. Before his appearance, Birt and Etho spend hours at the top of Sudden Hill pretending with their two cardboard boxes: “Sometimes they’re kings, soldiers, astronauts. Sometimes they’re pirates sailing wild seas and skies.’’ It’s cheap, profound fun, and when Shu, who has been patiently watching, asks to join, “Etho smiles and says, ‘Sure!’ ’’ Birt, though, sits apart from the other two, hands on his knees as the other two happily share a snack. And when Birt’s not smiling, Davies draws him without any mouth at all.

The detail perfectly illustrates Davies’s gifts (if you haven’t read his lovely, moving “Storm Whale,’’ please get on it). The children he draws — round-faced and snub-nosed — are very cute, but he never condescends to them. He captures their postures with charming precision. From spread to spread, Davies shows the friends’ bodies moving between solemn purpose and kinetic joy. He and Sarah manage to give life to small moments and big feelings among the friends.

There’s no dramatic incident that pushes them apart, yet as Shu joins the crew, disrupting Birt and Etho’s “two-by-two rhythm,’’ readers see Birt drift first into sadness and then into anger fierce enough to drive him from Sudden Hill. Though Shu and Etho keep calling on their friend, Birt resists. It takes a spectacular display of engineering, imagination, and cardboard to draw Birt back into their orbit and for the friends to develop a “three-by-three rhythm.’’ After many pages of generously observed scenes and language that is both subtle and sharp, Davies and Sarah don’t need any tricks to pull readers along; there’s big emotional payoff to this narrative ride.

NICOLE LAMY