Although it is perhaps not CS Lewis's very best book (which would probably be, according to taste, The Screwtape Letters, That Hideous Strength, Abolition of Man, or the Narnia Chronicles), his fable The Great Divorce is perhaps the one that I find comes to mind more often than any other.
You can find a copy here.
The reason I think of it frequently, is that TGD is the wisest of books concerning the most significant, yet difficult, of Christian doctrines for modern people - the nature of, and necessity for, repentance of sins. In particular, that The Problem for salvation (the choice between Heaven and Hell) is not the size of a sin (how sinful it is), but whether a person is prepared to recognise and acknowledge a particular sin as a sin.
Thus, a repentant murderer is in Heaven; an insincere Bishop prefers to remain in Hell.
A further value of TGD is that it shows exactly and plausibly why a 'normal', everyday person might actively-choose Hell, and for reasons that would perhaps be regarded as utterly trivial by another.
The title of 'The Great Divorce' has always been the book's biggest problem - since it is both off-putting and misleading. In fact the book is an easy and enjoyable read, full of humour and satire - as well as poetry and visionary fantasy, along similar lines to The Screwtape Letters. It is also manageably brief (about 150 pages).
If you want to know a bit more before giving the book a try, I can recommend Adam Greenwood's article; which discusses the book from a Mormon Christian perspective.
But why not just read the thing!
