There are countless rare books in the world, but by most expert's standards the rarest of them all is the Gutenberg Bible. It was the first book ever printed back in 1456, and although several hundred copies were originally printed finding a complete first edition would net you $25-$35 million. In today's market single pages alone go for $25,000 each, and several years ago just 1 volume (it's a 2 volume set) sold for $5.5 million.
Besides the Gutenberg Bible other rare books include the first edition of Shakespeare's collected works from 1623 (worth $6 million), a collection of Leonardo da Vinci's manuscripts (potentially worth as much as $100 million), and of course any first edition copy of the Declaration of Independence (worth $8 million). Edgar Allen Poe's work also makes in onto the "rarest books" list with his first published poem ("Tamerlane") being rare and difficult to find due to a byline of simply "By A Bostonian." It's worth as much as $200,000 to the right buyer.