There are many reasons why I regard this book so highly.
The writing style and narrative is enjoyable, flowing, well-paced, accessible, exciting. The book is superbly researched. The events and human subjects covered in the book are interesting, anything but dull, the authors uncovered uncommon stories about them.
Most of all, I felt like I had walked away after reading this book seeing these spacefarers and astronauts as humans, real people, not celebrities. The authors managed to show us their humanity without losing respect for their accomplishments. In fact, in spite of their humanity, their weaknesses, their environment, I have even more respect for so many of these spacefarers now that I can appreciate what they went through and had to overcome to achieve what they were able to.
I even came away with a much deeper appreciation and understanding of people I've often considered enemies at worst (i.e., Russian spacefarers), and objects of derision at best.
The stories in this book touch the human spirit in a way that is universal, beyond politics, beyond creed, beyond country. That's because the authors were able to capture an underlying essence most humans on our planet share: the curiosity and wonder to explore and give one's life to a cause greater than one's own agenda or paradigm.