The Paradise War by Stephen R Lawhead
Wolves prowling the streets of Oxford. A Green Man haunting the Highlands ...Drawn from the dreaming spires of Oxford to the misty moors and glens of Scotland, Lewis expects little more than a pleasant weekend away. But the road north leads to a mystical crossroads, and he finds himself in a place where two worlds meet, in the time-between-times. The ancient Celts admitted no separation between this world and the Otherworld: the two were delicately interwoven, each dependent on the other. In The Paradise War this balance is disturbed - a breach has opened between the worlds and cosmic catastrophe threatens.