Though the book is on its face an adventure story, a major theme is the need for a love of education; through this, Milo applies what he has learned in school, advances in his personal development, and learns to love the life that previously bored him. Critics have compared its appeal to that of Lewis Carroll's "Alice's Adventures in Wonderland" and to L. Frank Baum's "The Wonderful Wizard of Oz". Additionally, Maurice Sendak, in his introductory "An Appreciation" included in editions of the book since 1996, quotes a critic as comparing "The Phantom Tollbooth" to Bunyan's "Pilgrim's Progress": "As "Pilgrim’s Progress" is concerned with the awakening of the sluggardly spirit, "The Phantom Tollbooth" is concerned with the awakening of the lazy mind."