The Wonderling Mira Bartók 450 pp. Candlewick Press. $21.99. (Middle grade; ages 8 to 12)

One of the chief pleasures of children’s books is the central role friendship so often plays — a lonely character moves from isolation and misunderstanding toward the warmth and security of true kinship with others. It’s immensely satisfying to watch young people take control of their own fates, navigating outer peril and inner struggle in order to find connection, as do the protagonists of two enchanting new middle grade books.

The hero of “The Wonderling,” Mira Bartók’s debut children’s novel, is a groundling, a human and animal hybrid, known only by the number stamped on the tag that hangs around his neck. This sensitive soul, part boy, part fox, has only known the grim confines of Miss Carbunkle’s Home for Wayward and Misbegotten Creatures, a joyless place where music is forbidden and the only greenery is the moss growing on the stone wall that surrounds it. He doesn’t know who he is or where he comes from, and — most poignantly — wonders why he has been born at all.

After he is befriended and rechristened Arthur by a plucky fellow groundling, Trinket, the friends manage to break out of their miserably cosseted world and plunge into the great unknown, the world that’s so vast he “had to close his eyes from time to time in order to adjust to its size.” He arrives in the shining city of Lumentown and then, in the most opulently strange and inventive passages of the book, descends to the underworld beneath the city, where the clanging from mines and factories echoes through fog “thick as soup and brown as umber,” slugs leave glowing silver trails in the darkness, and hundreds of crows’ eyes glitter like stars at the top of a chamber.

Too often the bullies in the story speak in stale clichés rehashed from the movies, and chance or magic can sweep in too conveniently to save the day. But these weaknesses don’t diminish the power of the grand set pieces and exhilarating twists, or the pleasure of humble, exquisite moments. A home inside a tree smells like “pine wreaths, cedar chests, rosemary and mushrooms from deep within the woods.” Soon after their escape, Arthur and Trinket settle in a mossy spot between tree roots and gaze up at the night sky: “A cool wind blew, and the two friends pulled the blanket tight around them.” Momentum builds toward a thrilling crescendo and, rarest of all, a wholly satisfying ending that still whets the appetite for a sequel.