I read my first Michael McIntosh book in 81. "The Best Shotguns ever made in America". He really spoke to me in his writings and I have been a fan ever since. I like Gene Hill, as did Michael.
Michael thought equally well of Steve Smiths writings, as do I.
Bob Brister gave me a thorough understanding of shotgun ballistics in his book, Shotgunning, The Art and The Science.
I was never a big Fan of Don Zutz. He wrote about shotguns with all the flair of a treatise on bus transmissions but if you view shotguns simply as tools, you may feel differently.