The Boy and the Beast novel is written by Mamoru Hosoda.

Tokyo's a big city--big enough that it's easy for a grieving boy like Ren to slip through the cracks. But Ren slips a little farther than he meant to, ending up in the beast world of Jutengai! He ends up with a new name--Kyuta--and a new life as the apprentice of the bear Kumatetsu, learning the way of the sword. Kumatetsu's got problems of his own, though, and the boy and the beast may have more to learn from each other than they realize. But the boy's arrival in the realm of the beasts has put both their worlds in danger, and they're going to need more than life lessons to face it!