Sometimes it depends on the book. Is the book a happy one with a happy ending? That usually does it for me. I am a sucker for happy endings. :)
Sometimes mystery books make me happy. I become curious and want to find out what happens (or happened I guess). It's like watching an episode of Monk on USA Network!
I was reading something that RandomBuzz had posted on it's main page here. The main question was: Do you choose to read tearjerkers? Or do they make you too sad? When I read good books that make me shed a tear or two, it's usually because someone died. (When Sirious Black died- Oh, don't get me started on that!) But if one of the characters you like dies, did reading that book make you happy? It's a hard question.
Can you deal with one of your favorite characters dying?