You know a book's funny if you're reading it alone and laughing hysterically. Of course, the really great comic masterpieces are also deep reflections on humanity and life and death (like Huckleberry Finn or All Creatures Great and Small or Calvin & Hobbes). Frequently, though, you just need to guffaw, and the literary quality of the piece isn't quite as important as the laugh-factor.
Not that you want to read swill. Cheap laughs might be funny on the surface, but the jokes that actually say something are generally funnier than the ones that simply point and snigger. It's easy to think we're superior because we get the joke, but in reality a lot of modern comedy is no more than the absence of reverence. Real comedy is simply an incongruous presentation of a familiar idea that amuses with its absurdity while offering a new view.
Before you dismiss the need for humor on religious grounds (suggesting that Jesus is the Man of Sorrows, and so must we be in order to be like Him), remember that the "laughter is good medicine" image comes from the Bible (Proverbs 17:22), and that as the Creator of everything God invented laughter. He invented jokes. As the perfect man, Jesus probably told the best jokes ever.
We have a few of them—a rich person will enter heaven as easily as a camel getting through the eye of a needle, or the one about the Pharisee and the plebe who go into the temple at the same time to pray. Jesus clearly saw the absurdity of life (in both of the cases mentioned, the absurdity of human pride), and used it to direct people to the Author of Meaning.
Basically, laughter is really good and you should try it sometime. If you're a little rusty and need a boost, read a funny book: maybe the heartwarming but hysterical adventures of the Gilbreth clan in Cheaper by the Dozen, or the pure zaniness of Alice's Adventures in Wonderland, or the heartwarming zaniness of Jeeves and Wooster. Whatever you do, smile more, frown less, and for goodness' sake take laughter seriously.