Rose, Alexander. Washington's Spies: the Story of America's First Spy Ring. NY: Bantam Books, 2006. HG, BU, MA 973.385
It's fairly common knowledge at this point that George Washington was not America's best general ever. He wasn't our best-ever strategist or tactician, and he came close at several points during the Revolution to losing it all due to strategic or tactical errors. But it's long been known that old George was quite good at one thing, certainly-- learning from his mistakes.
It seems, however, that scholars of the period have discovered that George was good at something else as well. Not just good, in fact: he was something of a visionary when it came to intelligence gathering and espionage. Now, things started badly for Washington here as in so many of his endeavors-- initial forays into the "spy game" resulted in blunders like the Nathan Hale affair, in which schoolteacher turned spy Hale was sent into the lion's den to gather intelligence. He had nearly succeeded despite the overwhelming odds, only to be taken into custody when he mistakenly flagged down a British frigate instead of the colonial navy ship he had been expecting. Washington was thus left hanging (as, alas, was poor Nathan) while a new agent was recruited.
Washington soon hit on the idea of using spies who remained in place behind enemy lines, and were thus less conspicuous to the British and local Loyalists, instead of spies sent in from outside who stood out like sore thumbs as they observed British activities and attempted to question locals and British soldiers about British numbers, movements, and armaments. The problem then was getting out messages from spies to the First Spook-- a problem solved by the use of, among other things, invisible ink.
Washington's (largely successful) effort to build what modern-day spooks would call an all-source intelligence network was one of the main reasons that he was able, with a poorly equipped and nearly always outnumbered army, to defeat the British and help a fledgling nation gain its independence. Read this book, and you'll discover how this most remarkable man accomplished this most remarkable feat.