Codex Leicester

The Codex Leicester, written by Leonard Da Vinci, is currently the most expensive book ever sold. This journal contains the artist's notes on the connection between art and science, accompanied by his drawings and sketches. The manuscript is handwritten in Italian on 18 folded sheets of paper, creating a 72-page document.

In 1994, Bill Gates, the founder of Microsoft, purchased this highly sought-after item for $30.8 million, equivalent to almost $44 million today. Gates shares this masterpiece with the world by displaying it in various locations each year. Additionally, he had some of the sketches scanned and digitally recreated for use as screensavers and wallpaper through Microsoft Windows, allowing a wider audience to enjoy the images.

Gospels of Henry the Lion

The Gospels of Henry the Lion, believed to have originated around 1188, is considered one of the most expensive books in the world. This ancient manuscript consists of 266 pages and contains four gospels along with 50 miniature paintings that depict Henry's pursuit of power as the ruler of Brunswick. The Monks of the Benedictine monastery Helmarshausen wrote this manuscript upon Henry's request. In 1983, it was sold through Sotheby's London for a staggering $12 million, which would be equivalent to nearly $25.5 million in today's currency.

Birds of America

The four-volume Birds of America by artist and author James Audubon played a key role in familiarizing people worldwide with the variety of bird species in nature. Featuring over 400 paintings, Audubon utilized these illustrations to produce hand-painted engraved plates that were marketed as sets from 1827 to 1838. The entire collection, presented in a rare bound folio format, was auctioned in New York in 2000 for $8.8 million, which would be nearly $11 million today when adjusted for inflation.

First Folio: Comedies, Histories and Tragedies

William Shakespeare, one of the world's greatest playwrights, deserves a spot on this list. His First Folio: Comedies, Histories and Tragedies, published in 1623 after his death, set a record for the price it fetched at auction in 2006. This manuscript contains over a dozen of Shakespeare's plays, including his most famous works and some previously unpublished stories. Only 750 copies of this book were ever produced, and this particular copy is now owned by private collectors. Originally sold for 20 shillings, it is now valued at close to $5.5 million, based on its 2006 auction price of $5.2 million at Sotheby's in London.

 Cosmography

One of the oldest and rarest books is a copy of the first atlas ever printed, dating back to 1477 and known as Cosmography. This work is based on the research of the Greek mathematician, geographer, and astronomer Ptolemy, who lived in the second century. What makes this book, along with other early atlases, valuable is that each map was hand-etched onto a printing plate. Today, only two copies of this book are held by private collectors. One of them was last sold in 2006 for $4 million, which, adjusted for inflation, equals more than $4.2 million today.