Cover art
Allston, Aaron. Doc Sidhe. Baen 1995.

Allston, Aaron. Doc Sidhe.
What if: Thousands of years ago, Earth separated into a "grim world" and a "fair world", with magic taking a prominent place in the latter.
Summary: Trying to prevent his girlfriend's kidnaping, a kick-boxer from our Earth, the grim world, is accidentally transported to Neckerdam, the fair world's analog of New York City.

Published: Baen 1995 (0671876627).
Web link: Baen Free Library
Bookstore: Listed by Amazon.com in paperback.

Allston, Aaron. Sidhe-Devil.
Series note: Sequel to Doc Sidhe.
Summary: Doc Sidhe and his associates fight an international conspiracy of fair-folk racists.

Published: Baen 2001 (0671319930).
Bookstore: Listed by Amazon.com in paperback.