IMO-- Bob Brister was a technician who covered all the shotgunning bases, and was a master story teller and a first class shot as well- Ditto for Mike M., but with the caveat that he covered double guns of both "persuasions". Don Zutz had his own ideas, in some ways like Francis Sell and "Cactus Jack" O'Connor, all made their mark in the world of shotgunnery and will be missed.
That being said, the three late gun writers who influenced both my interest in shooting shotguns effectively on game (not clays so much) were Nash Buckingham, Paul A. Curtis and especially, Gene Hill. When you read one of Gene's articles on shotguns and shooting, you felt as if he was standing right next to you, lending helpful advice gained from years of actual experience.