Andre Norton 
Dragon Magic 

Four boys from different races and nationalities are drawn together by a potent magic that hurtles them into worlds populated by heroes and dragons Sig, Artie, Kim, and Ras go to the same school but have nothing in common—except for the urge to explore an abandoned house in the neighborhood. Who knows what treasures they’ll unearth? In one of the rooms is a dusty old box with four pictures of dragons on top, containing pieces of a jigsaw puzzle all jumbled together. But they glitter like jewels and lead each boy back to an enchanted time.   Sig becomes a Viking warrior who must slay a dragon who had once been a man and now guards a cursed treasure. Ras is a Nubian prince sold into captivity; he can only escape by killing a deadly Egyptian serpent. As Artos, named for Caesar, High King of Britain, Artie lives under the yoke of Saxon tyranny, waging war to defend King Arthur and the Pendragon flag. Now a sword bearer and page in the imperial palaces of the great Chinese emperor, Kim must follow the path of the slumbering dragon. Against a backdrop of myth and fantasy, acclaimed author Andre Norton weaves an enthralling tale of friendship, faith, loyalty, and heroism.  Dragon Magic is the 4th book in the Magic Sequence, but you may enjoy reading the series in any order.