Non Fiction for me, especially books dealing with the study of mammal behaviors.
I am a big proponent and user of public libraries laden with books and equipped with computers and printers available for homework.
I feel that reference and textbooks should be available in e-book format because in book form they are so heavy and expensive.
This way public libraries would have more than one reference book and would be able to lend the reference and the e-texbooks.
Some of my favorites:
Never Cry Wolf: Farley Mowat
Gorillas in the Mist: Dianne Fossey
The Year of the Gorilla: George Schaller
Almost Human: A Journey into the World of Baboons by Shirley C. Strum
In Quest of the Sacred Baboon: Hans Kummer
Most Frans de Waal books except the newest ones (hoping to catch up)
Elephants Memories: Cynthia Moss