Take the bus and see the sights. It's worth waiting in line.
There are few activities more charged with the potential for peril (or adventure, apparently) than standing in line. So, at least, is the experience of the nameless protagonist of C.S. Lewis's vision of the hereafter. He boards a bus only to discover that he has been in the Shadowlands all along; as he and his fellow tourists disembark hours later, they are given a vision of Heaven, though it is not a tame country.
Instead, he finds that the very grass will pierce his feet and that an apple weighs more than a boulder. The various souls he encounters discourse on topics ranging from free will to the very definition of love. Most significant is his run-in with his teacher among the "Solid People", the author George MacDonald.
How will he return to speak of what he has seen? How quickly will understanding dawn, and will it be in time enough? Lewis's arguments, as always, are well-reasoned, and he couches philosophy and theology in the guise of fiction.
Good is good and evil is evil, and never the twain shall meet. If anything, both become more clear as the story progresses, and they become more and more distinct from each other all the time.
The journey into Heaven and Hell is allegory in the very best sense of the term, and Lewis makes no attempt to hide the Christian nature of his work. Lewis, of course, comes from a liturgical Anglican tradition, but there is little (if anything), that other denominations would find objectionable.
For those who don't share Lewis's Christianity, however, he may come across as heavy-handed. The Great Divorce makes no claims to be anything other than what it is: insight into heavy spiritual truths.
Those stalks of grass can really hurt...and watch out for the unicorns.
Drug and Alcohol Content
None to speak of.
There is a strong hint that one of the protagonist's companions has engaged in some sort of sexually deviant behavior (symbolized by a lizard attached to his shoulder).
Lewis's intent, however, is not to glorify sin but to show how it can be either transformed, or, if it is not, what the dire consequences are.
Crude or Profane Language or Content
Usage of "damn" and one or two uses of "bloody"; also, one ghost takes God's name in vain - a subject addressed by one of the spirits and shown as incorrect. Sin is ugly, and Lewis doesn't hold back. The souls of the damned exhibit the range of all seven deadly sins.
There is a reason why Lewis has transcended his own time, and The Great Divorce illustrates how a skillful author can deal with difficult issues and still be readable. It would be nearly impossible to read this particular example of his craft without having to think.