Hello, Hello! is a nonfiction picture book which describes how different animals greet each other when they meet in the wild. Elephants, for example, are "very, very, very, very friendly," and say hello by putting the tips of their trunks in one another's mouths!
This is one of the best nonfiction books I've found for preschoolers. It reads like a storybook and contains clear, accurate, but not extraneous information. It worked amazingly in my "Say Hello" storytime, although you could use it for a variety of themes.