What book does Montag want a copy of?

Montag wants a duplicate copy of the Bible.

"Professor Faber, I have a rather odd question to ask. How many copies of the Bible are left in this country?"

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Montag showed her a book. "This is the Old and New Testament, and-"

"Don't start that again!"

"It might be the last copy in this part of the world."

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"There's only one thing to do," he said. "Some time before tonight when I give the book to Beatty, I've got to have a duplicate made."