Ursu gently teases out the tale of a close-knit family whose youngest child, James, vanishes into thin air -- literally. (A circus magician makes the kid -- poof! -- disappear, then can't bring him back.) Househusband Justin and his doctor wife, Hannah, find their blissful world suddenly frail and full of child-size holes. But James' sister, 7-year-old Greta, reaches into her imagination to locate her brother: ''Lots of times when there's a mystery, there's a clue, too, and it's up to the girl to figure it out, because adults just don't.'' Ursu's story is sweetly done, her own compelling magic trick.