Books you'd describe as being "a dark Narnia"

Sounds like Inception. Not a book, but it’s so close to the premise you described in the OP that I figured it was still worth a mention.

I wouldn’t call either the books or the writer “atheist.”. They’re downright anti-theist. G-d exists, all right, and he’s a right miserable bastard, and institutions that support him (literally) are corrupt and terrible.

About as subtle as a brick to the head, by the end of the series.

And, since I missed replying to this the first time around:

Oh yes, it gets a *lot *bleaker and darker.

Certainly - that’s my point: it is often claimed to be an atheist book, but it isn’t. In a real “atheist book”, one would hardly expect to find an afterlife (even a horrible one).

Boo hiss! I was going to say this one. I’ve read most of the others (The Magicians, Dark Materials, etc). I would say this comes the closest. It is a dark book - Dopers: if you are a fan of the rest of the novels listed in this thread, don’t miss this one.

I haven’t read the Keys to the Kingdom books, but I was going to suggest the *Abhorsen *series (also known as the Old Kingdom series) as being of possible interest. The first one, Sabriel, is probably the closest to what the OP describes. The heroine is actually from the fantasy realm of the Old Kingdom, but spends most of her youth at a boarding school in the mundane country of Ancelstierre. (This is a fictional place and not quite the real world, but magic exists only near the Old Kingdom border and the Ancelstierrians have early 20th century technology.) The two worlds share a physical border and it’s possible to walk back and forth between them, but it’s suggested that they are in different planes or dimensions. Different stars are visible in the two kingdoms, and the passage of time doesn’t quite sync up.

The fantasy world in this series is quite dark, rather more so than I expected from a YA series. The heroine comes from a line of Abhorsens, magicians who act as reverse necromancers and send ghosts and revenants back to death, so she spends a lot of time dealing with the undead.

You…appear to be mocking the Wiz.

I’m going to need you to take that back.

Taking another aspect of the OP (people not adapting well to coming back to the real world), there’s Jo Walton’s short story “Relentlessly Mundane”, which is “the kids who came back from Narnia, five years later, and how they adapted to not saving the world any more.” The end feels like kind of a cheat, but what can you do.

Well, there are parts of Oz that are wicked dark.

There are parts of Wicked that are oz dark?

Seriously, if you haven’t read Maguire’s Oz books, do so. They are so much better, rich, and darker than the musical.

Clive Barker’s “The Thief of Always” has an alternate world of sorts that is very dark indeed - though it doesn’t seem so at first. Juvenile fiction, but I actually like it better than his adult work - I think Barker works better when he’s forced to work within limits on his more outré interests.

Pan’s Labyrinth is probably disqualified being a movie, but man, that movie’s harsh.

I came out of the theatre wanting to believe in the good interpretation of the ending.