Johannes Gutenberg experimented with movable type throughout the 1440s, using his own casting process to set type from an alloy he invented. He had printed some relatively unremarkable items before working on the Bible, which he first offered for sale in 1455 [source: University of Texas at Austin]. It is believed that 180 copies of the Bible were produced -- 135 on paper and 35 on vellum, or calfskin. Today, only 48 copies are known to exist, some of them only partial, almost all of them in the hands of museums, universities and libraries [source: Clausen Books].
Gutenberg Bibles are rare and valuable for a number of reasons. In addition to their scarcity and status as the first of their kind, they're books of exceptional quality. Gutenberg used finely crafted paper and vellum, as well as a special ink of his own invention that has remained vivid for centuries. Most of the Bibles were sold in folio form, or as loose pages that the owner would then have bound in the style of his or her desire. That gives each Gutenberg Bible a certain uniqueness. Each Bible is usually split into two volumes, with the Old Testament filling the first volume, and the second volume containing the remainder of the Old Testament and the entire New Testament.
How much is a Gutenberg Bible worth? A perfect copy hasn't changed hands in years, but in 2007, a single leaf went on sale for $74,000. In 1978, Christie's auctioned off a perfect two-volume Bible for $2.2 million. Another Christie's auction in 1987 transferred an incomplete version to a Japanese company for $4.9 million [source: Clausen Books]. As long ago as 1897, a perfect vellum Bible sold for $20,000 [source: NY Times].