[Update: I'm required by the new FTC guidelines to tell you that this post includes Amazon Affiliate links. I choose these books myself, from the local library or my own bookshelf.]
City Dog, Country Frog is such a sweet story.
A city dog and a country frog become friends. They teach each other their games and stories. They spend every day together...until, one winter day, the frog is gone. The dog waits and waits for the frog, but the frog never returns. The dog is lonely, until the next spring, when the dog makes a new friend.
I like this book for a few reasons. One, the watercolor illustrations. (Duh...when do I ever not like illustrations?) The pictures totally match the tone of the story. Two, the story implies that the frog has died during the winter, but it doesn't actually say so--which makes it easy to discuss the frog's disappearance on your child's level. And three, I'm always a sucker for stories about unlikely friendships. Like I said before, they're sweet.