Primary Sources:

Reproductions or transcriptions of 1,140 relevant documents can be found in L. Kinvin Wroth, ed., Province in Rebellion: A Documentary History of the Founding of the Commonwealth of Massachusetts, 1774-1775 (Cambridge: Harvard University Press, 1975). Local libraries that list this source include:
Holy Cross
Clark
Worchester State College
National Heritage Museum, Lexington
Mt. Holyoke, Williams, Amherst
Hampshire
Framingham
Tufts, MIT, Harvard, Boston College, Brandeis
(Before heading to one of these libraries, make sure they have the microfiche documents, not just the short, bound volume that gives only a surface summary.)

Several revealing letters can be found in Thomas Gage, Correspondence of General Thomas Gage, Clarence E. Carter, ed., (New Haven: Yale University Press, 1931).

For British assessments of patriot strength in Worcester the following spring, see Allen French, General Gage’s Informers (New York: Greenwood Press, 1968), 15, and “General Gage’s Instructions, of 22d February, 1775, to Captain Brown and Ensign D’Bernicre,” and “Narrative, &c.,” MHS, Collections 4 (1916): 204-218.

Relevant secondary sources:

Ray Raphael, The First American Revolution: Before Lexington and Concord (New York: The New Press, 2002. The September 6 event in Worcester is featured on pp. 130-38 and 242-244.

Ray Raphael, “Blacksmith Timothy Bigelow and the Massachusetts Revolution of 1774,” in Alfred F. Young, Gary B. Nash, and Ray Raphael, eds., Revolutionary Founders: Rebels, Radicals, and Reformers in the Making of the Nation (New York: Alfred A. Knopf, 2011), 35-52.

Dirk Hoerder, Crowd Action in Revolutionary Massachusetts, 1765-1780 (New York: Academic Press, 1977).

Richard D. Brown, Revolutionary Politics in Massachusetts: The Boston Committee of Correspondence and the Towns, 1772-1774 (New York: W. W. Norton, 1976).

Ellery B. Crane, Services of Colonel Timothy Bigelow in the War of the American Revolution, including his 15th Regt. Massachusetts Line in the Continental Army (Worcester: Blanchard Press, 1910). Compiled from oral histories, not to be trusted in every detail.

Ellery B. Crane, “A Chapter in the War of the American Revolution,” Worcester Society of Antiquity, Proceedings, 25 (1909-1911). Compiled from oral histories, not to be trusted in every detail.

Justin Florence, “Minutemen for Months: The Making of an American Revolutionary Army before George Washington, April 2—July 2, 1775,” Proceedings of the American Antiquarian Society, 113:1 (2003).
http://www.americanantiquarian.org/proceedings/44539557.pdf

Worcester Histories, some with transcribed documents:

John L. Brooke, The Heart of the Commonwealth: Society and Political Culture in Worcester County, Massachusetts, 1713-1861 (Amherst: University of Massachusetts Press, 1989).

Kenneth J. Moynihan, A History of Worcester, 1674-1848 (Charleston: History Press, 2007).

Donald E. Johnson, Worcester in the War for Independence (Clark University: PhD Thesis, 1953).

William Lincoln, History of Worcester, Massachusetts, from its Earliest Settlement to September, 1836 (Worcester: Charles Hersey, 1862).

Albert A. Lovell, Worcester in the War of the Revolution: Embracing the Acts of the Town from 1765 to 1783 Inclusive (Worcester: Tyler & Seagrave, 1876).

Lillian E. Newfield, Worcester on the Eve of the Revolution (Clark University: MA Thesis, 1941).

Charles Nutt, History of Worcester and Its People (New York: Lewis Historical Publishing Co., 1919).

Caleb A. Wall, Reminiscences of Worcester from the Earliest Period… (Worcester: Tyler & Seagrave, 1877).