John James Audubon's Birds of America
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Inside Audubon's Birds, the world's 'most expensive' book

A rare copy of what is billed as the world's most expensive book is being sold at auction in London on Tuesday.

John James Audubon's Birds of America was the natural history publishing sensation of the 19th century.

The book is valued at between £4m and £6m and is being sold as part of a collection from the estate of the 2nd Baron Hesketh.

Leslie Overstreet, a curator of natural history rare books at the Smithsonian Insititution in Washington DC, explained why this giant book is so important.