Most historically important books for improving captive animal welfare?

[La Cucaracha]

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.

 

[15399]

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.