Joe Towson's post was a good summary of the Haymarket Square incident.
For the literary angle (which was exclusively my man Howells) see:
Garlin, Sender. _Three American Radicals_ (Boulder: Westview
Press, 1991), 97-143 [derivative but focussed].
Cady, Edwin H. _The Realist at War_ (Syracuse UP, 1958), 69-80.
Lynn, Kenneth S. _William Dean Howells: An American Life_ (New
York: Harcourt Brace Jovanovich, 1970), 288-292.
Also the beginning of the "Theoretical Socialists" chapter in
Smith and Gibson's _MT-Howells Letters_.
Cady reprints in full the long editorial Howells wrote but evidently did
not send to the New York Tribune after four of the anarchists were
executed. Lynn reprints the letter to the editor of the Tribune in
which Howells urged the governor of Illinois to commute the death
penalty. The file of newspaper clippings Howells collected about the
incident is with his papers in the Houghton Library at Harvard.
Possibly this is more than most readers of the MT Forum want to know, but
it's a fascinating case. And only sorta like Oklahoma City.
Kansas State Univ.