The hunting ones I can take or leave, make a good read but I don't feel the need to keep them or re-read.

Cardew - From Trigger To Target.
Hiller - Collectors Guide To Air Rifles and Air Pistols
Beeman - Blue Book Of Airguns

All books that I try to keep to hand for quick reference. I also have a copy of Wesley - Air Guns And Air Pistols kicking around.