Apparently some lurid tell-all biography of Redgrave was published in the UK a couple years ago. This is definitely not that book. In fact the title is a bit of a misnomer because really it should be "The Art of VM" or something like that, as it's basically a long critical essay about her career, with Callahan obviously a major (but not blind) fan who's tried to see everything he possibly can. (He's missed a fair amount of her stage work, and a few obscure film/TV projects are basically inaccessible or even lost, but apart from that he's seen it all.) He's a bit snobbish about anything he deems unworthy of her (meaning pretty much any blatantly commercial enterprise), but his opinions are well articulated even if you might disagree with some. (I can't believe he found her performance in Ralph Fiennes' "Coriolanus" underwhelming, for one thing.)
The "life" part gets a lot less attention, but then Redgrave didn't grant an interview for this book, the people close to her are dead or similarly close-lipped, and colleagues who do comment mostly offer statements of professional awe--as well they might. (Even Meryl Streep admits being intimidated the couple times she's briefly worked with Redgrave, not because the latter is imperious but because she's just...so...amazing.) Redgrave's own memoir published 20 years ago wasn't very revealing about the way her mind works, either. Both books confirm she's fairly humorless (even though she's occasionally done quite well in comedy), that she is downright obsessively sincere about her many political causes over the years, and that as a result she's one of those people who can't see the trees for the forest. In other words, she's so sincerely bent on "saving the world" that she sometimes has little time or patience for the people actually in her life. (Although it's suggested here that she's mellowed a bit in recent years on that score.) She may be narrowly focused to a fault at times, but she's hardly self-centered--if anything she's the definition of "bleeding heart liberal," always concerned with other people's suffering and injustices. It's noted here that the reason she's made a lot of dubious "career choices" lately (i.e. voice acting for cartoons, et al.) is because she's always given her money at the drop of a hat to whatever cause she perceives as needy, and despite living very modestly for someone of her stature, basically needs to keep making the rent.
So, she comes off as an admirable if not necessarily all that likable person. Callahan does his best trying to define the alchemy that can make her performances so extraordinary, though it's hard to explain magic. (It is interesting to read about how spontaneous and non-"technical" she is as an actor, sometimes maddening her directors and fellow cast with her impulsive new ideas, especially in stage productions.)
The book does delve into her oft-controversial politics and her complex family/romantic relationships--it HAS to, because they often directly impacted her career. But if you're looking for a lot of dirt, look elsewhere. Redgrave has lived a very, sometimes uncomfortably public life, but she's intensely private in some ways, and appears to be one of those people who just doesn't like talking about herself--she finds it not only invasive but frivolous. Perhaps the main thing preventing this from being a great book is that its subject, as "well-known" as she is (some would say notorious), remains in many ways unknowable. She's no open book, probably even to herself.