by Jessica Martini, Christina Bossinakis and Jill Williams LEXINGTON, KY - The Keeneland September Yearling Sale continued to churn out double-digit year-on-year gains as its second Book 2 session concluded Thursday night with yet another son of Gun Runner leading the way. Through four sessions of the 12-day auction, 671 yearlings have grossed $307,639,000. At this same point a year ago, 640 head had sold for $252,528,000. Asked about the gross surpassing the $300-million mark after day four, Keeneland Senior Director of Sales Operations Cormac Breathnach laughed and said, "It's...