The Codex Leicester, Leonardo da Vinci — $30.8 million
The most famous of da Vinci’s scientific journals, the 72-page notebook is filled with the great thinker’s handwritten musings and theories on everything from fossils to the movement of water to what makes the moon glow. The manuscript was first purchased in 1717 by Thomas Coke, who later became the Earl of Leicester, and then, in 1980, bought from the Leicester estate by art collector Armand Hammer (whose name the manuscript bore for the fourteen years he owned it). In 1994, Bill Gates nabbed the journal at auction for $30,800,000, making it the most expensive book ever purchased. But hey, at least Gates put his purchase to good use — he had the book scanned and turned into a screensaver distributed with Microsoft Plus! for Windows 95.