You would be hard pressed to find a credible source to declare that Reagan was not suffering from the early stages of dementia/Alzheimers during his last two or three years of office . . .
Believe what you want. It doesn't take much effort to find plenty of articles citing information contradicting what O'Reilly has in his book. It is his schtick. He patted himself on the back for not interviewing anyone who might have actually seen Reagan after his "killing." And he promotes his books endlessly on a show I no longer watch but if you like him, fine and dandy.
I subscribe to the doofus line of thought.