Another favorite artist and author of picture books is Arnold Lobel. Owl at Home, Frog and Toad, Mouse Soup, etc... you might think they are simple books from the "I Can Read" series, being inexpensive paperbacks with few colors, but behold the beauty inside!
Owl at Home
Owl at Home
Frog and Toad
I also loved Arnold's wife Anita Lobel's work, particularly Princess Furball.