Frog and Toad Together by Arnold Lobel
Amanda Pig, Schoolgirl by Jean Van Leeuwen, illustrated by Ann Schweninger
Arthur's Honey Bear and other Arthur titles by Lillian Hoban
Commander Toad by Jane Yolen
Tom the TV Cat by Joan Heilbroner
Sleepy Dog by Harriet Ziefert, illustrated by Norman Gorbaty
Tiger Is a Scaredy Cat by Joan Phillips, illustrated by Norman Gorbaty
Mouse Soup by Arnold Lobel
You may notice that the first four titles come from series. I listed my all-time favorite from each series. I insist that you read all of the books about Frog & Toad, Amanda, and Arthur.
Please note that Arthur the chimpanzee is not to be confused with the aardvark of the same name. They are completely different characters from different series, authors, and illustrators.
I've also posted this list at Amazon Listmania.