I love Ezra Jack Keats' stories, and I'm afraid he's being forgotten. This is one of our family's favorites:
A Letter to Amy by Ezra Jack Keats
Ages 5 and up
There is no great Amazon description of this book, but I think it does a wonderful job of capturing the feelings of a child as they attempt to invite a friend to a birthday party. Especially when they're a boy and that friend is a girl.