Archer's Quest, by Linda Sue Park (Clarion 2006).
In this time travel fantasy, Chu-mong, the Great Archer of Ancient Korea, appears suddenly in Kevin's room, after having fallen off a tiger. Kevin has no idea how it happened, but knows that he must return Chu-mong to the past or history will be irrevocably altered.
This intelligent fantasy has Kevin seamlessly drawing on a wide array of knowledge as he attempts to acclimate Chu-mong to the present day and send him home. Archer's Quest is dramatic, fun, smart, and satisfying.