After a stirring e-mail exchange with his father, awardwinning essayist and cultural commentator Ilan Stavans decided to do something bizarre: revisit his hometown, Mexico City, accompanied by a tourist guide. But rather than seeking his roots in the neighborhood where he grew up, he headed to the Centro Historico, the downtown area at the heart of the world's largest metropolis. It was there that conversos, the hidden Jews escaping the might of the Holy Office of the Inquisition, were burned at the stake. And, centuries later, it was the same section where Jewish immigrants, both...