Ish is a story about a boy loving his artwork until he is criticized for not creating art that looks like what it is supposed to look like. This is a phenomena that happens gradually to most children as they get older. When young, children love to create art. But as they grow up they often get frustrated that their art doesn't look like what they see through their eyes. Many children begin to hate art as they go through elementary school because they become so self critical.
In this story the boy finds out that his sister has been secretly saving his art work because she enjoys how it looks something like what he is trying to portray but not exactly-It doesn't look like a vase-it is vase-ish. This concept frees the child to continue to create art without criticism. A good book for children of any age-including us grown ups who have often stopped creating art because we think we aren't good enough.