My DD is a science and animal lover who refuses most fiction. She is very sensitive and does not like to read anything about animals killing each other either. So, my recommendations are all fairly gentle non fiction.
James Herriot's Treasury for Children
Cows, Cats, and Kids: A Veterinarian's Family at Work
These are a great series in Animal Behavior:http://www.kidscanpress.com/US/Anima...ion=5&series=2
Seymour Simon has a ton of science books, including many animal ones: http://www.seymoursimon.com/
DD has been really into birthing this past year and liked this one: http://www.amazon.com/gp/product/184...ef=oss_product
Some fiction comments: Artie and Merlin is pretty gentle. Owlie is a fun graphic novel style. Catwings was a bit too sad for her. We are going to try Horse Diaries (there are several in the series) perhaps. I have one but I have to pre-read it first.
Hope that helps!