Encyclopaedias that cost £15 'worth £10,000'
The set, still in regular use by owner Charlotte Hampson and her family, was bought by her father in the 1970s for 15.
Mrs Hampson, of Chelmsford, Essex, came forward with her third edition set after Encyclopaedia Britannica launched a hunt for the oldest collection.
She believes her volumes came from a stately home in Suffolk but had been given to an estate worker who no longer needed them.
"My father bought the books over 30 years ago from a local acquaintance," she said.
"We had no idea that they were particularly rare or unusual but we've always loved them for their interesting contents and wonderful smell.
"My sister Joanna and I enjoyed browsing them as children."
Mrs Hampson's 1797 edition contains several quirky entries including
the one for "fool", which reads: "A fool is a person who makes false conclusions from right principles; a madman, on the contrary, draws right conclusions from wrong principles."
Mrs Hampson's set was one of 2,800 collections presented to experts after a prize worth 2,600 was offered to the owner of the oldest books brought forward.
The prize included the 2010 Limited Edition set, one year's access to Britannica online and a leather-bound replica of the first edition from 1771.