This deserves more than a capsule response, but I'm afraid I'm feeling wordpoor--so a couple quick thoughts to flesh out the vague stars.
At first I thought (feared) this was going to be a more conventional biography, as Powers zeroed in on certain elements of de Mello's early life and the book was structured chronologically . . . but it quickly revealed a more subtle focus, embedded in the development of de Mello's work and philosophy. Tracking S. V. de Mello allowed Powers to ground a substantive overview of the major problems of a truly global politics (at the end of the 20th century) in relation to the specific experiences of one part of the body nominally charged with enacting that truly global politics.
In a nutshell, his whole life grappled with the dilemma facing the United Nations, whether to pursue humanitarian efforts (the pragmatics of housing and feeding the huge populations of displaced, sweeping up in the aftermath of natural and human catastrophes) or human rights (a vision of social, economic, and political justice). Aiming at the former meant, usually, appeasing the demands of various direct and indirect state agents, sucking up to this powerful nation, watching relief efforts get crippled by an inability to take any kind of side in a local conflict. On the other hand, the idealized purities of a human rights agenda could lead not just to the disabling but actual dismantling of any kind of practical effects on those masses affected by but unrepresented in inter-State politics. Getting people rice and bread, while shaking hands with brutal dictators? Or refusing to soften a sharp critique of genocidal practices, while people starve? The book is far more complicated than that neat summary suggests, and sticking (mostly) to detailed and energized accounts of de Mello's various UN efforts, Powers builds a very nuanced understanding of the enormous difficulties burdening any sense of a non-State global political agency.
And, like her protagonist, she builds a very effective case for a pragmatic vision of social justice, attuned to the practical and the ideal -- always in negotiation with the specific tensions of current history, of the local's relationship to the global.
It's a heckuva good book. At times, I wondered why--in a book really about that "theory" of geopolitical justice--I would get details about de Mello's love of wine and women. At other times, the converse: why not more about this fascinating man? But I don't think the story could be told in the abstract, as 'mere' theory or even just as history; by grounding the book in one man's actions, her (and his) concern for the necessary vision of Justice in a practical historical moment becomes the reader's problem, too. Highly recommended.