In no particular oder:
The Shotgun Handbook by Mike George
Vintage Guns for the Modern Shot by Diggory Hadoke
Winchester Shotguns & Shotshells by Stadt
For wonderful pics Winchester Shotguns by Dennis Adler
Winchester's Finest the Model 21 by Schwing
Winchester An American Legend by RL Wilson
The History of Winchester Firearms by Thomas Henshaw
The Better Shot by Ken Davies'
Shotgunnin g The Art and the Science by Brister
Shotguns & Cartridges for Game & Clays by Gought Thomas
The Shotgun A shooting instructor's handbook by Michael Yardley
EVERYTHING written by Don Zutz
Modern Sporting Guns by Austyn
Heyday of the Shotgun by David Baker
Everything by Geoffrey Boothroyd
Everything by Michael McIntosh
Oops, tired of typing.....
Ask Michael Petrov he has extensive what to have list.

If I was forced to own one book it would be Shotguns & Cartridges for game & clays by Garwood.
If I had to own only one gun it would be Winchester Model 12. Mine is used for: fowling, turkey, deer, and property defense when laser designator/super light module is attached to the magazine tube. The 3" version is fearsome weapon even when empty!