[timgilbert]

That's... kind of a rubbish summary, but I'm not sure what I'd want a ~700-word summary of The Great Gatsby to look like.

[LeonardoTolstoy]

Honestly, I think you could do worse than the Wikipedia summary. That is maybe a bit too long (I can't remember the Wikipedia guidelines for books) it looks longer than 700 words. But from there I think you could fashion a solid 700 word summary of the book without too much trouble.