Mike McIntosh's stuff "Shotguns and Shooting," "More Shotguns and Shooting" and especially his work with Dave T, "Shotgun Technicana" are all worth reading and having in a gun library.

I love the classics by Gough Thomas, "The Gun Book" etc.

Bob Brister's "Shotgunning: The Art and the Science" is also interesting more for "practical shooting stuff".

I'm curious to see Diggory Hadoke's new book, "VINTAGE GUNS FOR THE MODERN SHOT"

Hope this helps!