Romans too! Glad we don't have to pick! However, the persecuted church does. They get one page of the Bible and have to pass it along to other people. We can help get Bibles into their hands though! Go here and put one into their hands!
It would have to be one of the Gospels. Without the "good news" the rest of the Bible is opaque. I would hate to lose even one book, but I can't imagine being deprived of the story of Jesus. With just one Gospel (Matthew or Luke), I could stand amazed at His birth, watch His early life of quiet obedience, hear the Father proclaim pleasure at His baptism, learn from His words, be silenced by His miracles, be comforted by His compassion, be corrected by His rebukes, identify with His disciples of little faith, agonize and rejoice at His wounding and crucifixion, exult in His resurrection, and take courage at His Great Commission.