Most historically important books for improving captive animal welfare?

Currently reading Heini Hediger's Studies of the psychology and behaviour of captive animals in zoos and circuses and Wild Animals in Captivity. Also reading David Hancocks' A Different Nature.
I'm wondering what other books have been historically important for modernizing captive animal welfare and modern zoo design.

 

Anything written by Marian Stamp Dawkins would be a good start. She's considered by many to be a pioneer in the scientific field of Animal Welfare, albeit her focus was more geared towards domestics, much of her ideas apply to any animals kept in captivity.