Black by Ted Dekker

Enter an adrenaline-laced epic where dreams and reality collide.
Fleeing assailants through alleyways in Denver late one night, Thomas Hunter narrowly escapes to the roof of an industrial building. Then a silent bullet from the night clips his head and his world goes black.
Now Thomas wakes from a deep sleep, remembering the vivid dream he just had of being chased. Incredibly real. His head is even bleeding - but he's fallen on a rock. He's in a green forest, waiting to meet Rachelle, the woman he's falling madly in love with.
That night, Thomas tumbles into bed and falls into a fitful sleep. He dreams. But here comes the real mind bender. Every time Thomas falls asleep in one reality, he awakes in the other. He truly no longer knows which reality is real. Each reality has dramatic impact on the other, each proves to be real, each presents huge stakes, and the fate of each will depend on one man: Thomas Hunter.