Some resources

Second Nature: Environmental Enrichment for Captive Animals by David J. Shepherdson, Jill D. Mellen (Editors)
Wild Mammals in Captivity : Principles and Techniques by Devra G. Kleiman, Mary E. Allen, Katerina V. Thompson, Susan Lumpkin (Editors)
Environmental Enrichment For Captive Animals by Young, Rob J. Young, Robert John Young
A Different Nature: The Paradoxical World of Zoos and Their Uncertain Future by David Hancocks
Reading Zoos: Representations of Animals and Captivity by Randy Malamud
Zoo Culture by Bob Mullan, Garry Marvin