I actually read that book a few years ago. As I recall, the evidence was rather circumstantial. There were some movies made a number of years ago, one with Richard Basehart, with a Freudian twist suggesting Hitler may have had Oedipus Complex. As Hitler was beaten frequently by his drunken father, it is natural that he became very close to his mother who offered love and protection. Stalin too, came from a very similar type of background, brutally beaten as a child by a drunken father. No wonder both men became such callous brutes as adults.
It isn't the easiest read due to all the citations. Not a book one can expect to get through on a rainy weekend.
As I recall, the evidence was rather circumstantial.
Yes indeed and the author makes plain that his case is circumstantial.
It's worth noting that reviews of the book by leftist publications don't question the accuracy of the author's research. They instead generally try to be dismissive and toned down... as if not wishing their review to draw attention to the book or subject.
However, when you look at the preponderance of evidence including, Hitler's occultism, his hatred of Judeo-Christianity, curious behavior during the Anschluss consistent with that of a man trying to hide something from his past, his artsy background, his artsy associates, that early nazi meeting halls were also known "gay bars", his life within a fraternity of men and the number of bisexual/homosexual men near him, what other plausible explanations are there?