Cuckoo

The fight to survive is all in the mind. 

There is nothing exceptional about Greg Summers, until the day he returns home to discover that his wife no longer recognises him and is married to a stranger using his name. 
Perhaps it is an elaborate hoax, yet that wouldn’t explain his vivid flashbacks to childhood, or the violent eruptions of blood that accompany them. Nor does it explain the stray memories that seem to belong to an entirely separate man called Richard Jameson. 

One of these men is a lie, and neither wishes it to be he. On the run from a creature that cannot exist, his comfortable truths shattered, Greg finds his whole knowledge of the world questionable. If he does not know himself, what can he trust himself to know? 

"A plot as wild as this could have easily spun out of control, but Wright holds the reins tight. His dexterity is dazzling." - Hellnotes