John James Audubon was a naturalist and painter whose love of the natural world has made his name virtually synonymous with the act of observing and learning about wildlife. In fact, the Audubon Society is named after him. In 1820, he began painting birds in an attempt to paint every single avian species in North America. Audubon eventually completed 435 life-size bird paintings; Robert Havell Jr. engraved the works. To accommodate the size of the paintings, the engravings were printed on the largest sized paper available at the time -- known as double-elephant -- leading to the collection's nickname, "The Double-Elephant Folio."
The paintings weren't issued in book form, but rather sent to subscribers, several at a time. This is partly why so few complete copies remain -- about 120, by most counts. They're often collected and bound into a series of volumes. A full set would've cost about $1,000 when they were printed. Today, individual plates sell for thousands of dollars, and a full collection sold in 2000 for $8.8 million, making it one of the most expensive printed books in the world [source: Princeton Audubon].
The New York Historical Society has all the original watercolor paintings produced by Audubon for the book.