If you were told that the books would end tragically before you'd read it, it wouldn't be an enjoyable. There'd be no suspense, because you know it'll end in tragedy.
Not bad. Worse, I would say.
It doesn't ruin the story, but I still think it worsens it. If I know what's going to happen, there's no suspense, and if I know it's a tragedy, I won't bother getting emotionally invested in characters. Also, you re-read books?
Storygames are different, because they change when you re-read them.