Probably A Christmas Carol, but there's also Goodbye, Mr. Chips, Dr. Jekyll and Mr. Hyde, The War of the Worlds, The Time Machine, Bartleby, A Boy and His Dog, Kuttner and Moore's Vintage Season, Coraline, the four Sherlock Holmes novels...
'Time Bomb'
Literary Buffistas 3: Don't Parse the Blurb, Dear.
There's more to life than watching Buffy the Vampire Slayer! No. Really, there is! Honestly! Here's a place for Buffistas to come and discuss what it is they're reading, their favorite authors and poets. "Geez. Crack a book sometime."
Of what hasn't already been mentioned (Vintage Season is without peer, as is just about anything by Kuttner and/or Moore):
Jane Austen's Lady Susan is a fine short epistolary novel.
James Clavell's King Rat is probably too long to really qualify, but it's a brilliant depiction of life in a WWII Japanese POW camp.
Bel Kaufman's Up the Down Staircase looks longer than it reads. It's another epistolary (more or less) novel of a young teacher's first semester out of college, teaching in a NYC public school.
Is a short novel the same as a novella? I love Colette's Gigi (never seen the film.)
Favorite short novels would be:
Bonjour Tristesse, Francoise Sagan
The Haunting of Hill House and We Have Always Lived in the Castle, Shirley Jackson
Of the top of my head, Shopgirl by Steve Martin and Who Will Run the Frog Hospital? by Lorrie Moore come to mind.
Maybe Lansdale's Mad Dog Summer.
From looking at lists of novellas, I think the format doesn't work for me very well -- there are certainly some I like, but I prize things I can reread, and with novella length fiction I'm probably going to remember most of the details, unlike with a novel, but it'll still take a while to get through, unlike a short story. So even if I'm like, "Oh yes, that was good," I'm rarely in the mood for them.
I liked The Body Artist and it worked well in the short format.
So what do you like in the short form?
Tea With the Black Dragon -- R. A. MacAvoy
Tea With the Black Dragon
Lovely book, though I wouldn't call it a novella.
Too long?