I have to agree with GA Russell. I thought the ending made no sense. Especially when one considers that the whole story is supposedly told from the point of view of an aged Dr. Sheehan from his memory. To much of the story just does not fit that.
It was effective as a thriller right up until the ending, but that ending made no sense. If the whole story was an elaborate fantasy created by Dr. Cawley and Dr. Sheehan, as well as all the other staff that would have been required to go along seemingly being a part of the elaborate secret experiments while dropping warning hints to Edward/Andrew, what was the purpose? Alternatively if all of that was just in Andrew's imagination, what of it really happened? Meeting Rachel in the cave? Destroying Dr. Cawley's car. His conversation with the warden? All of these details were later related to Dr. Sheehan by Andrew so that Dr Sheehan can write the memoir? To seize on one particular incident near the end where Andrew 'shoots' Dr. Cawley. Supposedly Andrew really was in the military and then a Federal marshal, yet he mistakes a squirt gun for a real revolver? Not going to happen.