Sophie's Squash by Pat Zietlow Miller, illustrated by Anne Wilsdorf (Schwartz & Wade, 2013).
This is a really fun book about a girl and her unusual pet, a squash named Bernice. The illustrations are so playful and quirky; they fit the tone of the story perfectly. Ultimately Sophie's pet doesn't last, but she plants her in the ground and come summer she has lots of baby squash for friends.
Random House has some activities to go along with the book here.