The Trumpeter of Krakow (paperback) Eric P. Kelly
There is something about the Great Tarnov Crystal.... Wise men speak of it in hushed tones. Others are ready to kill for it. And now a murderous Tartar chief is bent on possessing it. But despite this, Joseph Charnetski is bound by an ancient oath to protect the jewel at all costs.
When Joseph and his family seek refuge in medieval Krakow, they are caught up in the plots and intrigues of alchemists, hypnotists, and a dark messenger of evil. Will Joseph be able to protect the crystal—and the city—from the plundering Tartars?