Exegetes and expositors no doubt wish that Lewis had been a more precise interpreter. Rationalists no doubt wish he had stuck with explaining what is real. But I think Lewis' brilliance is his ability to use fantasy and mythology to speak of things more real than could properly be exposited or explained.
To read this book for facts about Heaven and Hell would be a mistake. To read this as an exposition of the afterlife, I think, would be a mistake. This book, I believe, is an exposition of the great gospel truth: "He who loses his life, finds it." Lewis is illustrating real life, real love, and real happiness. And the key is self-forgetfulness. Those who fail to enter Heaven are those who cannot get over themselves - their ambition, their suffering, their rights, and so on. Those who enter, who become more real and experiences life more fully than they could imagine, are those who come into the light to have their shame painfully exposed and lose themselves in love for God and others.
Put down your self-help books! Your discontentment comes not because you have failed to consider yourselves. You are unhappy not because you have not attended properly to your own happiness. It is precisely your grasping, which keeps you from it. You are dying because you have not lost yourself. You have not yet forgotten yourself.