Nutmeg and Barley: a Budding Friendship by Janie Bynum. 

Nutmeg is a chatty, active squirrel who happens to live next to Barley, a shy and quiet mouse.  They make some little steps toward friendship, but many of them go awry.  It isn’t until Barley gets very ill and Nutmeg worries that she hasn’t seen him that the friendship is firmly started.  The illustrations of this story are wonderful.  They beautifully capture the world that these two animals live in.  The story is also the perfect length to share with children in a story time. 

Add this one to your list of great friendship books!