1. The Codex Leicester: This is a scientific manuscript by Leonardo da Vinci, written around 1510. It was sold in 1994 at auction for $30.8 million to Bill Gates. Its high price is due to its historical significance and its unique insights into da Vinci's thought processes.
2. The Bay Psalm Book: This is a 1640 book of Psalms, the first book ever printed in what is now the United States. It is considered the most valuable printed book in America, with several copies fetching multi-million dollar prices at auction.
Arguments for other books:
* Gutenberg Bible: While not the most expensive in recent times, it is considered the most important book printed with movable type. There are only 48 complete copies known to exist, and many have sold for millions.
* The First Folio of Shakespeare's Plays: This collection of Shakespeare's works is also highly valuable, with copies fetching millions.
Ultimately, the "most expensive" title can change as time goes on and different books come up for auction. However, the Codex Leicester and the Bay Psalm Book are considered the most valuable books in the world, based on their historical significance and rarity.