Biographers have incentives to portray their subjects sympathetically. So when the current authors say that Rooney was unpleasant, neglectful and disloyal, then it's hard to disagree. It may seem contradictory to then say that their portrayal is sympathetic and admiring. But this is because they have only praise for Rooney's skill and dedication on stage or camera (at least, until he was into his 90s). Laurence Olivier, Cary Grant and James Mason were among those who rated Rooney as the best actor of his time.
The authors have got a surprisingly postmodern take on his character, or lack of one. "In the empty pit where Mickey's personality should have been, there was only a hollow echo of narcissism." Their theory is that, having been performing since before his second birthday, there was no way for him to develop a personality beyond whatever role he was currently playing. It didn't help having an alcoholic father, a mother who turned tricks in their Los Angeles house to make ends meet, and studio employers who, when he was still a child, put him on uppers and downers to squeeze as many films out of him as possible.
He made his last film in 2014, more than 90 years after taking to the stage, having lost tens of millions of dollars by gambling, manipulative agents, hopeless business schemes and, allegedly, elder abuse by relatives.
The authors do a good job overall, but let themselves down in minor ways, e.g. in terms of proofreading (Jack Paar's name is spelt two different ways on the same line) and in repeatedly plugging their former book on Max Jacobson ("Dr Feelgood").