A Letter to Amy

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A Letter to Amy is a 1968 children's picture book by American author and illustrator Ezra Jack Keats.
"Peter, from Keats' The Snowy Day, is having a birthday party, and he's asked all of his friends to come. But Amy is a special friend because she is a girl--so Peter decides to write her a special invitation. When he rushes out in a thunderstorm to mail it, though, he bumps smack into Amy herself and knocks her to the ground. Will she ever come to his party now? And how will the boys greet a girl?"