Stock Fitter's Bible by Rollin Oswald
This book is new and goes into detail on how all the dimensions of a shotgun effect your shooting. I have read a fair amount of shotgun shooting books over the last couple years since im fairly new to the sport. This was the best book i've read because it pinpointed the effects each dimension of a shotgun will cause.
For example, cant is relative to the height of the trigger hand elbow. If the elbow is too low this can result in twisting the gun causing either shooting to low on station 5 right angle birds, or rainbowing to compensate for this. Another point was made that the lenght of pull (POL) is effected by stance, placement of feet, at each station. Also included the importance of extending the offhand index finger parallel to the barrel. The placement of the offhand on the forearm, this distance relative to the balance hinge point can effect the swing on straight aways. He also wrote about using light pressure in pulling your hands in opposite directions to create stability. this I have not tried yet and would like feedback from someone who has. Good reading and good luck for the coming year.