From the article:

"In this revelatory book, Begley shows the dark side of this showman. He disliked competitors, possessed an outsized ego, could not abide criticism and was prone to extreme exaggeration. He was also an ardent self-promoter. "

As far as dark sides of historical figures go, that doesn't sound so bad. I really expected way worse. He actually sounds like many public figures today, not sure I'd consider those things much of a dark side now.