I recommend reading this before you watch the documentary with the same title. Despite being written after and because of that film - the director asked the author if he was interested in writing it - it has, naturally more information, which makes easier to understand the movie (that has one or two moments when the facts are not chronologicaly); also, the documentary will play as a resume of what you just read. I do recommend both because they give you insights almost exclusive of each format: the book has more information and better, well elaborated, explanations about Nina's behaviors and feelings; the film can actually show you how she played and performed, how she talked and who were all these people mentioned in the book, all these important "secondary" actors.
A good biography, I suppose, ultimately will help you understand human nature, more than anything else. And will give you almost literally the thoughts and emotions of other people - whether you share a culture or other things with them, or not. This gives us a different perspective, the world through someone else's eyes - not somebody who is fictional, but a real person. It's a great tool to be more understanding and accepting of others, which is a very human thing to do - otherwise how could we live in a society? Not just this is important, this empathy, to build a healthy social structure, but also it is essential to keep it's components - people - happy, or at least satisfied. Nina was somewhat a misfit, not just because she was black and a woman, but because of all that she was - a black female artist who protested against social injustice; a pianist trained to play classical music, who crossed several musical genders in her songs, and expected the deserved recognition for her excelence; an artist who refused to just entertain; a free spirit, maybe, specially in the closed society where she grew up, who just wanted to love and be loved. I identify with a lot of that, in a way, and I hope others can do to, for their sake - to not feel alone, mostly - and for hers - she was really a bigger than life performer and musician, who should not be forgotten. Despite, in the end, we have an explanation for her erratic, sometimes violent and rude behavior, I still think there's space for criticism - she's still responsible for some of her actions, considering that she new, at times, that something was wrong and that she needed to change or a brake; she could have gotten help sooner, I suppose. The author does not give a subjective opinion, allowing the reader to develop his/her own.
I do hope you read it, or see the documentary; but mostly I hope you will listen to her music. Because it's not outdated, not in form nor in content - that, I may say, is unfortunate, that there's still injustice. Today there's all kind of genres with solid admirers, even if not much, so her music, I'm sure, can find it's niche (s) of fans. Considering how some of them reflected her times - she said "An artist's duty, as far as I'm concerned, is to reflect the times" - the experience of nowing her work is also a History lesson, and allows you to compare between then and now. I agree with her statement: music is a cultural "invention", a language to bond people, inspire people - which Nina wanted to do - to give them courage to act for change, but also preserve the past and teach the future - to prevent oblivion and perpetuate a knowledge that can be useful. “The artist is the creator of beautiful things", as Oscar Wilde said, but should also "reflect the times".