As any devoted book collector knows, you can't really put a price on the immense pleasure that comes with owning a rare book or historic manuscript.
However, due to limited supply and growing demand from collectors and enthusiasts, the prices of the world's rarest books are incredibly steep.
This week, we look at the Top Five most expensive books in the world - and what people who were willing to spend whatever it took invested in them...
#5 William Shakespeare, First Folio: Comedies, Histories and Tragedies - $5.5m
It's no surprise that one of the world's greatest playwrights should feature in this list. The First Folio was, in fact, published in 1623, several years after the writer's death.
Within the manuscript is more than a dozen of Shakespeare's legendary plays, including some undiscovered works among the well-known classics. Only 750 copies of the First Folio were ever made, and this is one of the few in private hands.
Originally the book would have sold for about 20 shillings. Fast forward to 2010, and the book is estimated to be worth about $5.5m (it sold for a World Record $5.2m at Sotheby's in 2006).
#4 James Audubon, Birds of America - $8.8m
Featuring more than 400 engraved plates of rare birds, Audubon's four-part Birds of America was vital in introducing hithero unknown species to readers all over the world.
Audubon originally sold his hand-painted plates as sets between 1827-1838. These were eventually brought together and bound in a folio, and sold at a New York auction in 2000. Birds of America's final price soared all the way up to $8.8m (which would be about $11m today).
#3 The Magna Carta - $21.3m
First issued in 1215, the Magna Carta (or "Great Charter") had arguably the most significant influence on the extensive historical process which led to constitutional law in today's English speaking world.
A manuscript from the Magna Carta's first year of issue, set forth during the reign of King John, was sold at auction by Sotheby's in New York.
The only copy in private hands today, it auctioned to winning bidder David Rubinstein of the Carlyle Group for $21.3m. Rubinstein planned to return it to the National Archives.
#2 Gospels of Henry the Lion - $25.5m
Henry XII, Duke of Bavaria until 1180, apparently requested that his gospels be written by the Monks of the Benedictine monastery in Helmarshausen, Germany.
Two-hundred-and-sixty-six pages long, including four gospels and 50 miniature paintings, the manuscript - dated by experts to 1188 - chronicles Henry's quest to become ruler of Brunswick, and charted his career.
The work was last sold in 1983 through Sotheby's in London for $12m. That would be almost $25.5m today.
#1 Leonard Da Vinci, Codex Leicester - $44m
Codex Leicester is the most valuable manuscript in the world, containing Da Vinci's notes, drawings and sketches linking science and the arts.
Written in Italian, the manuscript itself comprises 18 paper sheets folded in half and double-sided, creating a 72-page document.
Fittingly, the world's most expensive book was purchased by the one-time world's richest man: Microsoft found Bill Gates, who bought Codex Leicester for $30.8m in 1994. With inflation, that value would be closer to $44m today.
Each year since, Gates has shared his acquisition with the rest of the world, exhibiting it in different locations; while also having some of Da Vinci's sketches scanned to be enjoyed digitally by audiences all over the world. Beat that, Steve Jobs...