Atomic Clocks of the National Bureau of Standards.
Boulder, CO: U.S. Department of Commerce, 1978.
C 13.2: At7/3
Illustrated pamphlet on the history of the atomic clock issued in the 1970s at the advent of the technology.
From Sundials to Atomic Clocks: Understanding Time and Frequency.
Gaithersburg, MD.: Department of Commerce. 1999
C 13.44: 155/999
Layperson's guide to time, timekeeping, and the uses of time information.
Be sure to check the government website maintained by the National Institute of Standards and Technology for much more useful information.