The Trumpeter of Krakow — Eric P. Kelly

There was something about the Great Tarnov Crystal.... Wise men spoke of it in hushed tones. Others were ready to kill for it. Now a murderous Tartar chief was bent on possessing it. But young Joseph Charnetski was 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?