The Story of Ferdinand, by Munro Leaf, illustrated by Robert Lawson, Viking, 1936.
Some books are truly timeless -- this is one of them. I loved Ferdinand when I was a child (and still as an adult), and my son loved Ferdinand tonight when we read it together. Maybe because we would love to sit under a favorite tree and smell the flowers.
The artwork is anything but simple, even though the drawing are only black and white. The illustrations are complex, and rich, and animated, and, in short, as perfect and timeless as the text.