NASA turned 50 in 2008, so this might seem a little dated. But the History Office has made available a download of the book NASA at 50. The description reads:
"The 50th anniversary of NASA on 1 October 2008 found an agency in the midst of deep transition. In the closing year of the presidency of George W. Bush, only a month before the presidential election and in the midst of a worldwide economic crisis, the Agency was implementing a new Vision for Space Exploration intended to return humans to the Moon, to proceed onward to Mars, and to study the cosmos beyond.
It was in this milieu that the History Division at NASA Headquarters commissioned oral history interviews to be undertaken with NASA senior management. This volume is the result and provides a snapshot of the thinking of NASA senior leadership on the occasion of its 50th anniversary and in the midst of these sea changes."
If you want to better understand how our space program developed and the thinking of some of its luminaries on the past, present, and future, it's available here.