Aside from Wikipedia and the other usual suspects, here's a real interesting starting place.
"Porter Rockwell was that most terrible instrument that can be handled by fanaticism; a powerful physical nature welded to a mind of very narrow perceptions, intense convictions, and changeless tenacity.
In his build he was a gladiator; in his humor a Yankee lumberman; in his memory a Bourbon; in his vengeance an Indian. A strange mixture, only to be found on the American continent."
—Fitz Hugh Ludlow, 1870.
Sounds like he and Holliday could have had some interesting conversations.
Impressive resume, amazed I've never run across his name in the course of my casual interest in Western gunsligners and lawmen.