Are the final sentences in this short piece effective or not?
Not having read the beginning of the piece, it's difficult for me to say what you're trying to achieve here.
Questions that occur to me:
Why is it important to say that the couch is blue? If you want the reader to see the couch, you could be more descriptive.
Even so – why? Does it matter what kind of couch it is?
Is it significant that he reads the WSJ rather than just a newspaper? If not, why say it?
What kind of sound did he hear? It seems odd that you use the generic/non-descriptive word "sound" here, when you made a point of telling us the color of the couch.
There isn't much build-up of suspense here; if the letter is significant, perhaps you could let us know what is going on in Jack's head as he hears the sound, goes to the door, and finds the letter.
If this is the end of a story, I am having trouble seeing the point of the couch, the newspaper, the sound, or the letter.
The sentences are short, choppy, and not very descriptive.
Jack is also a very generic protagonist name.
Because it is so common, it's hard to imagine anyone but an average guy.
Sorry to be so critical, but you haven't given much to go on here.
With only these few sentences, I would say that they are not effective.
They don't make me want to read more or arouse any curiosity in me about the character or the letter.
Original Sentences: "Jack sat on his blue couch and read the WallStreet Journal.
A sound came from the door.
He got up.
There it was – a letter.
(Assuming this is the very end of a story)
In keeping with what Holloway and Kapoor have said, we had better know by this time why blue, why WSJ, why is he reading the paper at all? We also need to know why the letter's arrival is important and what it means for Jack and how events would have turned out differently for him if it hadn't arrived.
A small point is that doors don't usually emit sounds.
"From the direction of the door" reads better.
"He got up" is either too much or too little.
If he can see the mailbox from the couch he doesn't need to get up.
If he does, it needs to be a little longer.
He gets up and walks from the living room to the door, in order to show him as he walks and in order to build some suspense: is the mail the hoped-for letter or just another advertising circular?
The other posters have made some good points, although I think some of the criticisms flow from the fact that we don't know what came before the excerpt given: IMHO, it's hard to judge a writer's style from such a short sample.
What seems more problematic to me, though, is that, if these are the final sentences, it's not obvious what–if anything–is resolved by them: and that tends to be the function of a story's ending.
Perhaps if we knew what had happened before, this example would carry more weight: but still, "There it was – a letter" seems a pretty underwhelming ending to a story.
Shouldn't you at least tell us what the letter says or if it contains something other than written words? After all, if getting the letter is important (and I assume it is, since it appears to be the climax of the story), isn't what's in the envelope the important thing, rather than the fact that "an envelope" simply arrives?
I think it needs more introductory information to be as dramatically effective as you are trying to make it.
"He had been thinking about the decision from his publisher on his latest manuscript.
Will this be the book that will break open his career?" Or, whatever plot or theme you are trying to build up.
a letter" is something rather pedestrian, not something unusual to come from your mail-slotted door.
I think you have to work more to build up a greater sense of anticipation and drama.
Now, I realize, you may not want to be showing your hand too much too early, but there needs to be something more to make it more intense and interesting.
You could make it more lively:
Jack sat in his couch (from the story we should already know what colour it is) His eyes were focused on an article in the Wall Street Journal.
He saw words, lots of words, that now meant nothing to him.
He heard someone at the door and a slight 'thud'.
He turned his head.
"It must rain", he thought.
The envelope was wet.