It's a non-fiction book pretty much through and through. The author writes with an eye for entertaining narrative and thus you could assume that small parts are embellished a bit for this reason but overall I haven't seen any criticism of the book that claims anything remotely important to be fictional. Martha Dodd (the ambassador's daughter) comes across as particularly foolish. First because she tries to defend the Nazi regime to her friends and then when a bit of light finally dawns on her about its nature, switches over to becoming a useful idiot for the equally vicious soviet regime, working as an NKVD informatn/agent. And based on third party accounts and the old NKVD documents from her informant file, describing her as a useful idiot (stealing a phrase from Lenin) doesn't seem to be far off the mark. One soviet handler's description of her is rather amusing: "She considers herself a Communist and claims to accept the party's program. In reality she is a typical representative of American bohemia, a sexually decayed woman ready to sleep with any handsome man."
Dodd herself met Hitler once through an introduction by a man who at the time was a close friend of the dictator, and her description of him was that he was "excessively gentle and modest in his manners"... Indeed...
I'm genuinely asking as I haven't read it and don't know anything about it.