Harris Greene's life is falling apart -- he loses both his manager as a kick-boxer and his fiancee Gaby -- when he stumbled on an attempt to kidnap her. He rescues her, but is trapped himself when the kidnappers jump to another world.
One like 1940s Earth, with a few elements from Irish folklore. You can even figure out which countries are which, though they go by different names. He falls in with Doc Sidhe and the Sidhe Foundation, doing good in the world. They find him interesting and disconcerting, especially when he proves that they speak English -- it's not some translation sideeffect of the shifting worlds. And they look into why the kidnappers were interested in Gaby. There's trouble behind that.
It involves the magical dangers of owing someone, a eight-legged stuffed horse, a wooden airplane, the dangers of iron, airships, inventing blue jeans, and more.