This is a very difficult question to answer as there are so many books to choose from and I don’t know what particular aspect of zoos most interests you; however, a few of my personal favourites follow.
I agree with “Paradoxurus”
• The Management of Wild Mammals in Captivity (Lee S. Crandall; 1964)
is probably my very favourite zoo book and is essential reading for any zoo enthusiast.
Other particular favourites of mine are:-
• The Zoological Gardens of Europe (C. V. A. Peel, 1903)
• British Zoos (Geoffrey Schomberg, 1957)
• The Rhinoceros in Captivity (L. C. Rookmaaker; 1998)
• Longevity of Mammals in Captivity from the Living Collections of the World (Richard Weigl; 2005)
As a Londoner, particularly interested in the history of London Zoo, I would have to include all the following in my personal favourites:-
• A Walk through the Zoological Gardens (F. G. Aflalo; 1900)
• A Record of Progress of the Zoological Society of London During the Nineteenth Century (1901)
• The Zoological Society of London : a Sketch of its Foundation and Development, and the Story of its Farm, Museum, Gardens, Menagerie and Library (Henry Scherren; 1905)
• A Centenary History of the Zoological Society of London (P. Chalmers Mitchell; 1929)
• List of the Vertebrated Animals Exhibited in the Gardens of the Zoological Society of London 1828 – 1927: Centenary Edition in 3 volumes (1929)
Vol 1 Mammals (by S. S. Flower)
Vol 2 Birds (by G. Carmichael Low)
Vol 3 Reptiles (by S. S. Flower); Amphibians (by Malcolm A. Smith); Fishes (by E.G. Boulenger)
• London Zoo from Old Photographs: 1852 – 1914 (John C. Edwards; 1st edition 1996; second edition 2012)
The last is an excellent book and there is sufficient difference between the first and second editions to make it worthwhile owning both.