A copy of the world's most expensive book is expected to sell for more than $7 million (£4.5 million) at auction in New York.

A rare first edition of John James Audobon's 'The Birds Of America' (1827-1838) will be auctioned at Christie's in New York on 20 January and is one of only 120 complete editions known to exist, most of which are in institutions.

The four-volume set of double-elephant folios, each over three feet in height, features 435 hand-painted engravings, which are still considered the "highest achievement in ornithological art".

Tim Bryars, of the Antiquarian Booksellers Association, said the set was an "extraordinary work".

"You have to see it in person to really appreciate it. It's enormous and just an extraordinary creation," he told International Business Times UK.

"A book like this, published in the way it was, is a wonderful piece of art - and an extremely expensive project to have embarked upon.

"This was published in parts over a decade, partly because of the time it took to engrave each plate, but also so that the publisher could afford to print it, recouping his money at each stage, and so that more people could afford to buy it in instalments.

"It's an amazing achievement and only a limited number of people could have afforded it, when it was new as well as now. We're used to a world where even an expensive art book is generally £50 or less, but that's a recent phenomenon."

The last complete copy of a first edition was sold at Sotheby's in London in December 2010 for$11.5 million, arecord for the most expensive printed book sold at auction. Bryars said a "similar reaction" may be expected at Christie's.

"The vendor will be hoping for at least two people who want it as bad as each other, but $7 million isn't far-fetched. It has a track record," he said.

"Whether the copy makes as much as the last one will depend partly on condition and provenance, but also, although the last copy appeared at auction only a year ago, potential bidders will be aware that they might not get another chance for a decade.

"It's a rare book and who knows when the next copy will come out of the woodwork."

The set due to be auctioned was purchased by the fourth Duke of Portland around 1838.

"Christie's is deeply honoured to be entrusted with the sale of the exemplary copy of Audobon's masterpiece, the finest colour plate book ever produced," said Francis Wahlgren, Christie's international head of books and manuscripts.

"The excellent condition of the entire set is a testament both to the loving care taken by generations of the Portland family, as well as to the quality of the book's production more than 170 years ago."

Bryars added that, while Audubon's work is at the "top end" of the rare book trade, he was constantly surprised by the quality of rare books that can still be bought for less money.

"It's a shame that often when people compare books to works of art and other antiques they seem to view books as a poor relation, when they are often wonderful pieces of history.

"That's what this book represents. It is a piece of incredible art and a showcase for the style of printing that could create a project of this size."