Reformation Trust has now published a new introduction that looks very helpful: Steve Lawson's The Gospel Focus of Charles Spurgeon. Spurgeonophile Phil Johnson writes:
I own at least three dozen different biographies of the prince of preachers, but Steve Lawson's new book on Charles Spurgeon will from now on have a key place of prominence in my short list of favorites. Dr. Lawson understands what made the great preacher's heart beat: it was the gospel, charged with a passion for the souls of lost people and kept steady by the doctrines of grace.
You can preview it below: