What book does Montag bring to the refugee camp?

Montag does not bring a physical book to the refugee camp. When Granger asks what he has to offer, Montag says he has part of the Book of Ecclesiastes and some of Revelation. These are books from the Bible. Montag adds that he has these texts partly (memorized) in his head. The refugees have one other man who knows the Book of Ecclesiastes but Granger says, "If anything should happen to Harris, you are the Book of Ecclesiastes." Montag will therefore become the book. 

"Ecclesiastes" translates to "Gatherer," "Teacher," or "Preacher." This is fitting considering the fact that these refugees try to gather, remember, and teach the great works of literature. Ecclesiastes is basically an autobiography. The narrator gives advice on how to live life. One of the messages is that, since death is inevitable for everyone, the best thing to do is to live and appreciate life. 

So I commended pleasure, for there is nothing good for a man under the sun except to eat and to drink and to be merry, and this will stand by him in his toils throughout the days of his life which God has given him under the sun. (Eccles. 8:15) 

Among other themes in Ecclesiastes is the notion that one should humble himself/herself before God, and in that humility, he/she should follows God's commandments and pursue wisdom. This is the honorable way to live: pursuing wisdom and appreciating life. There is a pessimistic tone to Ecclesiastes but this goes along with the teachings of doing one's best in life despite all of the bad things in the world. These notions apply to Montag in significant ways.