In researching After Yorktown, I found these resources especially helpful.
The American Revolution in Indian Country. Colin B. Calloway's book complements Graymont's (see below) with a well-written perspective from native eyes.
Encyclopedia of the American Revolution. Harold Selesky's update of Mark Boatner's classic reference. Anyone with any interest in the Revolution should own an edition. Boatner's earlier editions are available from used book stores, and most libraries have it.
Founders Online. A joint project of the National Archives and University of Virginia Press, the site has thousands of letters and other documents by founders, with an easy-to-use search engine.
Global Gazetteer of the American Revolution. John A. Robertson's magnum opus—and a work-in-progress—that has a comprehensive list and details of every skirmish and battle of the war.
The Iroquois in the American Revolution. Barbara Graymont's groundbreaking research on the Northeast's strongest native nation.
Journal of the American Revolution. Todd Andrlik, with a cast of prestigious historians, has quickly turned the Journal (founded in 2013) into the leading site for the Revolution. The articles are rigorously documented and well-written.
The Negro in the American Revolution. Benjamin Quarles' classic can often be found in used book stores.
Oatmeal for the Foxhounds. Although Marg Baskin no longer actively maintains her site, it has a wealth of information about Banastre Tarleton and other British officers.
The Papers of General Nathanael Greene. Thirteen brilliantly annotated volumes that follow the whigs' best tactical general, a Rhode Island ironmonger, without whom Yorktown might never have happened.
TOLIFAL, The On-Line Institute for Advanced Loyalist Studies, by Todd Braisted, has a wealth of resources about loyalists Americans.