A Tale of Two Worlds.
The format of this book goes back and forth between chapters about Earth circa World War I and chapters about a fantasy land. Eventually a character from one place portals to the other, fulfilling part of a long prophecy that some people don't want to come about. This book takes a while to get going because most of it it background information for the reader, showing us what the fantasy world is like and introducing our unwilling hero, Edward Exeter.
The fantasy world is very mountainous with long valleys here and there. They call them "vales" and each one seems to be different than the others, and there are a few different languages. They also believe in 5 gods: Father, Man, Woman, Maiden, ...and sorry I can't remember the fifth one. These gods are actually humans called "strangers" because they come from a different world, either Earth or another world completely. Strangers gain mana from their worshippers and use it to do miraculous things, which helps solidify their standing as gods. They are practically immortal and bored and so they play The Great Game, which is basically messing with people's lives, starting wars, inciting violence here and there, trying to one up each other.
But there's this prophecy about a Liberator who is going to totally change thing and turn everything upside down. Edward turns out to be the prophesized Liberator, though he doesn't want to be. He ends up fulfilling parts of the prophesies. There's a group of people, headed by a few gods, who want to stop the Liberator and they keep trying to kill him.
The series itself is good but this first book was slow reading for me. The 2nd book is much better so keep reading! Don't quit, give it a chance to show us Duncan's awesome world-building skills.