"If we insist on keeping Hell (or even earth) we shall not see Heaven: if we accept Heaven we shall not be able to retain even the smallest and most intimate souvenirs of Hell." 
 C.S. Lewis wrote this book in 1945 in response to Blake's "Marriage of Heaven and Hell," which held the belief of moral subjectivism, or moral relativity, (all roads lead to God.) This belief is still commonly held today and therefore Lewis' book still has much to offer to those who struggle with the sense of directionlessness that is caused by this philosophy. This theme may have been the main focus but it also contains many more insights on subjects related to human nature and the desires that keep us from obtaining "infinite happiness." Actually Lewis' book approaches this all in a very non threatening way, (a great read for a-religious, or non christian.) 
 It's a story of a dreamer who travels from hell/purgatory to heaven and witnesses the inner turmoil that takes place in peoples souls as they choose to keep their hell even upon being presented with the choice to enter into heaven. Paradise lost and Dante's Devine Comedy, of course, are other great books to compare this book with and read in conjunction with. Some of the scenes I found most interesting where the arguments that were made by those who were seeking to maintain their pride and their longings and rationalize away any reason for even wanting to see God and enter into his presence. It's easy to look into ourselves and recognize these same battles! Yikes! Anwyway, great book! 
 Some other great quotes: 
 "I wish I had never been born," she said. "What are we born for?" "For infinite happiness," said the Spirit. "You can step out into it at any moment..." 
 "Good beats upon the damned incessantly as sound waves beat on the ears of the deaf, but they cannot receive it. Their fists are clenched, their teeth are clenched, their eyes fast shut. First they will not, in the end they cannot, open their hands for gifts, or their mouth for food, or their eyes to see." 
 "That thing is Freedom: the gift whereby ye most resemble your Maker and are yourselves part of eternal reality." 
 "... hitherto you have experienced truth only with the abstract intellect. I will bring you where you can taste it like honey and be embraced by it as by a bridegroom. Your thirst shall be quenched.