A CLASSIC book of bird illustrations discovered at a country house on the Isle of Bute after being ''lost'' since 1909, sold for #5.5m at Christie's in New York, last night.

The four-volume first edition of The Birds of America by John James Audubon (1785-1851) was bought anonymously, well above its estimate of around #2m.

The work was sold by the present Lord Bute, previously international racing driver Johnny Dumfries who prefers to be known as Johnny Bute, and the money will be used to maintain and improve Mount Stuart.

Considered the finest colour plate work on ornithology ever produced, the book was issued in parts to a list of 161 original subscribers who included MP George Lane-Fox of Wetherby, Yorkshire.

His family sold it for #380 at Sotheby's in London in July 1909, and the book disappeared and was thought to have been lost until it was rediscovered in the Marquis of Bute's library at Mount Stuart House.

Sale room experts spotted the faint pencil inscription ''Fox'' on one of the pages and traced it back to the Sotheby's auction.

They found the book was bought by Bernard Quaritch, the London dealers, where further research in the company's cellar revealed it had sold it to the fourth Lord Bute for #585 in October 1911.

It had been in the library, unknown to the art world, ever since.

The discovery established a direct provenance which went back to one of the original subscribers, said Mr Felix de Marez Oynes, international head of Christie's book department.

''It is one of the finest complete first editions ever offered for sale,'' he said. ''The quality of the plates - the book's great appeal - is exceptionally fresh. There was a tremendous interest in it before the sale.