Cover art
Smith, L. Neil. The Crystal Empire. Tor 1989 (cover art by Michael Whelan).

Smith, L. Neil. The Crystal Empire
Divergence: 1349 CE
What if: Christendom was destroyed in 1349 when an attempt to ship plague-ridden rats to Saracen lands backfired disastrously.
Summary: In 2042, a Helvetic North American escorts a mission from the Saracen Caliph of Rome into the secretive, mysterious Aztec empire.
Published: Tor 1986 (031294070X), 1989 (0812554256BUY, 0812554264); Grafton 1988 (0586200436).