Considered as World's most expensive book, the rare first edition of John James Audubon's illustrated "The Birds of America" was auctioned at $7.9 million by Christie's auction house last week.

The pre-bid estimate was thought to be around $7-$10 million for the work.  The winning buyer was identified as an American collector who bid over the phone.   The set at Christie's was offered for sale by the heirs of the 4^th Duke of Portland.

Another complete first edition of "The Birds of America" was sold on December in London for $11.5 million, which made it the most expensive print book ever auctioned.

The book includes some 435 hand-colored, life-size prints of 497 bird species, made from engraved copper plates from Audubon's original watercolors.  The engraved plates were sold in a subscription series in England, Europe, and North America.

Experts estimate that only 200 complete first-edition copies were produced between 1827 to 1838.  Only 120 of those are known to exist today.