Look, no one knows better than I, who have published some 25 books, how easy it is to think you "thought of" something original when in fact you had read it in another source a long time ago and forgot that in fact you were dredging up words that you had internalized. All authors do it---it's like a musician thinking he has come up with an original lick only to hear a record played 20 years earlier that he had heard and forgotten.
The safe way to do that is to document the heck out of EVERYTHING. I probably over cite, because it isn't right to copy other's phrases or whatever.
But on the negative side, SA built a little history empire, which involved research assistants gathering and even writing some of his stuff, then him putting finishing touches on, and in that case, you absolutely cannot police it effectively.
As for the time spent with Ike, that's serious stuff, and his bio of Ike was in large part built on this face time. Isn't it ironic, then, that Ambrose, who probably did NOT have the face time with his subject, wrote an objective and fair treatment while Edmund Morris who DID have unprecedented access to Reagan, completely blew it by inserting fictional characters (himself) repeatedly throughout the book "Dutch"?
That sir is an excellent point.
See my other post about Ambrose. With your writing background, I see your vantage point and agree with what you said.