Martin Provensen was half of theauthor/illustrator team with his wife Alice who started their children's book career with Little Golden Books. They were particularly good at detailed multi-framed pages of animals such as those in Our Friends at Maple Hill Farm and The Mother Goose Book.
Fred Gwynne is also American and probably better known as an actor who starred in the TV series The Munsters, but he was also an author/illustrator of children's books. His most well-known is probably The King Who Rained, a book that looks at humorous homonyms. A small girl visualises the things her parents say getting them all wrong. The king reigns, becomes the king rains, barefeet becomes bear feet, footprints, foot prince etc.
And thirdly Australian Kathie Atkinson is a wildlife photographer whose photos have been used in a large number of children's nonfiction books. Her most recent books are a lovely series called Weird Wild and Wonderful.