What is Maurice Sendak's famous trilogy of books and what unifies them?

Maurice Sendak considers his books Where the Wild Things Are, In the Night Kitchen and Outside Over Thereto be a trilogy. In his book, The Art of Maurice Sendak, he writes that they "are all variations on the same theme: how children master various feelings — anger, boredom, fear, frustration, jealousy — and manage to come to grips with the realities of their lives."

In Where the Wild Things Are, Max becomes king of a land of monsters where he works out his anger at his mother, who has just sent him to bed without his supper. Mickey in In the Night Kitchen goes to a strange town, where he helps bakers find the milk they need to make the morning's cakes. And, in Outside Over There, a little girl deals with her jealousy of (and poor behavior towards) her younger brother by searching for him when he is kidnapped by goblins.