A good book for me is one where I'll crawl into the skins of the main characters. And where I don't want to finish the book as I'll "never" see those characters again. Problem is, if I think it's a good book, I'll finish it faster as I won't be able to put it down...
I don't mind if I know more, I'll even try to worn the character that's moving into danger! I can't stand dumb characters though.