While working on Dreiser's An American Tragedy, I've been reading the trial transcripts of Chester Gillette's murder trial online and have added some links to the Dreiser page. It's possible to see just how directly Dreiser quotes from the transcript.
The Historical Society of the Courts of the State of New York includes information on the real-life Chester Gillette (Clyde Griffith in the book) murder case that inspired An American Tragedy, including a webcast lecture on the case and trial transcripts. The first 450 pages or so of the transcript focus on selecting jurors, etc.; the statement of the case concerning Grace "Billy" Brown (the Roberta Alden figure) begins in Vol. 1, p. 481. These can be compared with Dreiser's treatment of the case in An American Tragedy.
Chester's letters to Billy, Vol. 1, p. 567
Billy's letters to Chester, Vol 1., p. 570
Witness hearing a cry at the time of Grace Brown's murder, Vol. 2, p. 89
Testimony about Chester's recollections of the crime, Vol. 2, pp. 150 and following
Chester Gillette's testimony, Vol. 3; his account of what happened, p. 24; p. 35, claims that he did not strike Grace Brown