This was bugging me last night. I read this book probably 30+ years ago. It was a fictional account written from the point of view of Hitler's doctor, but this story was embedded in an outer story set in the future as if someone was time-travelling to live through the doctor's eyes. Sorry that this is a very vague summary: the details are very fuzzy by now but I do recall that it was well-written. I think a British author.
Also, the doctor (Morell!) was describing how he was prescribing all these terrible drugs to Hitler without really caring about the effect they had on Hitler's health. For example Antigas pills which I believe contained strychnine. So there was quite some decent amount of fact behind the fiction.