Lamb: The Gospel According to Biff, Christ's Childhood Pal by Christopher Moore. Its pretty damn funny. Most of what Moore writes is. There is also Island of the Sequined Love Nun, Practical Demonkeeping, You Suck, The Stupidest Angel, The Lust Lizard of Meloncholy Cove and Fool.
Lamb can be viewed as sacrilegious if you are the religious sort. BUT...funny.