By Mo Willems
Gerald and Snake each work themselves into a panic that their best friend likes a new friend better. It's very funny, and it turns out fine.
Piggie has a new friend named Brian Bat, and the two of them are having a great time together. Gerald is glad about it for about a second. Then the worry kicks in: what if Piggie likes Brian more than she likes Gerald? Over on the other side, Brian's best friend Snake is tying himself into the same knot about Brian. So the two worriers fret and spiral while the two new friends just keep happily playing, and the whole thing lands on the plain relief that a friend making a new friend doesn't mean losing you.
The whole book is one small, very real fear said out loud: my friend found someone new, so maybe I don't matter to them anymore. Willems puts that in the mouth of a fretting elephant and makes it funny before it ever gets a chance to get sad. A child who's felt shoved aside when a friend paired off with somebody else will know the feeling on sight, and here they get to laugh at it instead of sit in it. It's also short and easy enough that a beginning reader can get through it on their own.