The Great Divorce by C.S.Lewis
Most of us probably read Narnia as little kids, and most of us probably adore the saga of talking animals and magical lands to this day. The only downside to the Narnia books, I would say, is that their luminosity has eclipsed the glory of C.S. Lewis' more adult works.
On the overall amazing literature scale, 10 being the Bible and 0 being Mein Kampf, I would rate The Great Divorce a 7.
In its own genre of philosophical/theological based fiction I would rate it an 8.
And on the heavy-light reading scale, ten being Crime and Punishment and 1 being If You Give a Pig a Pancake, I would give it a 7. Like I said, its a jaw dropper and a tear jerker, and no Christian can read this book and expect to come out it unmoved and unchanged.
At an average rate of 500wpm this book can be completed in about three or four hours. This is a great, quick read for someone suffering from summer intellectual lolls. You know, those moments (rare, but possible) when the student actually misses school.
For those wanting to go deeper and who don't mind a little philosophic reading, a great accompaniment to The Great Divorce would be Everything you Ever Wanted to Know About Heaven by Peter Kreeft.