Dragon Magic by Andre Norton
Sig, Artie, Kim, and Ras live in the same neighbourhood and go to the same school, but they have nothing in common. Until each of them sneaks into the old abandoned house on the corner and discovers the mysterious puzzle box covered with pictures of four dragons. Drawn by powerful magic, the boys find themselves bound together by a mystery that will transform them all-and transport them into worlds that are populated by heroes and dragons of lore. Sig finds himself in the body of a Viking smith's apprentice; Ras becomes a Nubian prince in Biblical times; Artie becomes a Welsh soldier for the true King Arthur; and Kim becomes the servant of an ancient Chinese warrior.