Poul Anderson 
Midsummer Tempest 
A Holger Danske Book

Somewhere, spinning through another universe is an Earth where a twist of fate, a revolution and a few early inventions have made a world quite unlike our own.It is a world where Cavaliers and Puritans battle with the aid of observation balloons and steam trains; where Oberon and Titania join forces with King Arthur to resist the Industrial Revolution; and where the future meshes with the past in the shape of Valeria, time traveller from New York.