The book has the original publisher's green cloth cover
A first edition of Charles Darwin's On the Origin of Species, which was kept on a toilet bookshelf, is to be sold.
It is hoped the book, which was bought about 40 years ago in a West Country shop for a few shillings, will reach £60,000 in Tuesday's auction.
The book was kept on a bookcase in a guest lavatory at the owner's family home in Oxfordshire.
Just 1,250 copies of the work were produced in 1859. In April one sold at auction in Norfolk for £35,000.
The book goes on sale on the 150th anniversary of its publication.
Christie's auction house said the son-in-law of the current owner was at a Darwin exhibition and recognised a picture of the book's spine.
Margaret Ford, head of books and manuscripts at Christie's, said the person who bought the book may not have realised its value.
She said: "It's incredibly important. He knew it [the book] was textually important.
"He maybe did not know how much of a bargain he was getting."
The revolutionary scientific work, which has the full title On the Origin of Species by Means of Natural Selection, has the original publisher's green cloth cover and gilt-decorated spine.
It is described by Christie's as "lightly bumped" around the corners.