In 1778, two years after the British forced the Continental Army out of New York City, George Washington and his subordinates organized a secret spy network to gather intelligence in Manhattan and Long Island. Known today as the "Culper Spy Ring,'? Patriots like Abraham Woodhull and Robert Townsend risked their lives to report on British military operations in the region. Vital reports clandestinely traveled from New York City across the East River to Setauket and were rowed on whaleboats across the Long Island Sound to the Connecticut shore. Using ciphers, codes and invisible ink, the spy ring exposed British plans to attack French forces at Newport and a plot to counterfeit American currency. Author Bill Bleyer corrects the record, examines the impact of George Washington's Long Island spy ring and identifies Revolutionary War sites that remain today.
Pretty drily written but extremely thorough, and rebuts incorrect information about the spy ring that has been promulgated in recent years. (A Fox News anchor, lying?! GASP.)
A good introduction to the subject, covering the formation and successes (and otherwise) of the American spy network during the Revolutionary War. There is a section at the back of existing sites connected with the spy ring that you can still visit.