I would say it's a 'darker' book in terms of its tone and atmosphere. This seems pretty intentional. Think about it: how many happy moments (a la Lucy) do you find in The Silver Chair. There's a brief respite at Cair Paravel, but even Eustace mars the happy mood with his grumblings. There's not a lot of description of happy moods in the book until they leave Underland.

Not sure what happy copy of SC some others here might have, but maybe I'd like to read it ;-).

It's not that we're meant to be groveling in pain along with the book, but we are supposed to feel a much greater relief when they resurface and find Narnia well, as well as be joyful to see Caspian in Aslan's country (a favorite character from VDT, right!)

Interesting discussion. A lot of good points have been made. I'm prone to agree with AslainIsOntheMove that Caspian's family life is an unhappy one. I wanted to expand on a theme I noticed throughout the books to explain why I think SC is darker than most of the other books.

The missing mother motif...

In MN Digory's mother is ill and the threat of her death hangs over Digory's head throughout the story.

In LWW the mother is absent due to the children being sent away and is unsatisfactorily replaced by the officious Mrs. Mcreedy.

In PC Mother Pevensie is again absent. Caspian is introduced as an orphan with a rather awful aunt standing in the place as his mother.

In HHB Shasta, like Caspian is introduced with a missing mother.

in VDT the Pevensie mother is still missing and Eustace refers to his mother as "Alberta" so even the Scrubb mother is "missing" in title.

In SC Lewis kills off Rillian's mother in the backstory much like he has in Caspian and Shasta's backstories. The difference is that this is a character we have actually met. In VDT she is the young, vibrant love interest for Caspian. Also her death spurrs Rillian's lust for revenge.

The dying/dead mother motif is a theme that increases in menace in each book in the series culminating in LB with the destruction of (motherland) Narnia. I don't think Tirian's mother is even mentioned, its just taken for granted that she is gone. The missing mother theme increases in intensity with SC because the character is someone whom we have been introduced to as a vibrant young adult in VDT. This forces thoughtful readers to contemplate on their own mortality.

I think its interesting to note that in our earth the mothers are generally absent or ill and that in Narnia the mothers are generally dead. So the deeper the books go into Narnia and or Caspian's family line, the darker the books get.

As a sidenote, Rillian is described in SC as looking like Hamlet. I think SC is the only book in the series to reference a Shakespearian tragedy.

Also, Caspian almost killed Drinian because Drinian told him to because Drinian said it was his fault that Rillian was gone. A lot of trouble and distressing things happened in this story.