Hewlett, Richard G. and Francis Duncan. Atomic Shield, 1947-1952. U.S. Atomic Energy Comission, 1972. The second volume of the three volume A History of the United States Atomic Energy Commission. Text in each PDF is fully searchable.
"Hewlett and Duncan - Atomic Shield (complete).pdf" contains the complete text and images from Atomic Shield. 12mb
"Hewlett and Duncan - Atomic Shield (figures only).pdf" contains hi-res (600dpi) scans of the images from Atomic Shield. 30mb