The hardback book is one of just 500 original copies of Harry Potter and the Philosopher's Stone which was released in 1997.
Book experts say the novel is in the best condition they have ever seen and could fetch £25,000 to £30,000 when it goes under the hammer on tomorrow.
Its careful owners had kept the book safely stored away in a briefcase at their home in Lancashire, which they unlocked with a code, to preserve its mint condition.
The couple, a pair of retired civil servants who wished to remain anonymous, said: "It's been locked away in a briefcase along with a first edition of Harry Potter and the Chamber of Secrets which is also up for auction.
"The plan was to keep them as family heirlooms, which is why my wife put them in a briefcase. It was to stop the pages turning yellow."
Jim Spencer, of Hansons Auctioneers, said he felt like he was "dealing in smuggled diamonds" when the owners brought the book to be valued.
It is the second first edition of Harry Potter and the Philosopher's Stone he has uncovered in a matter of months.
One found in Staffordshire made £28,000 at auction but he believes his latest find could fetch even more due to is condition and scarcity.
He said: "A 1997 first edition hardback of Harry Potter and the Philosopher's Stone is the holy grail for collectors as so few were printed.
"To find another one so soon after the July sale has knocked my socks off.
"The owners took such great care of their precious cargo they brought it to me in a briefcase, which they unlocked with a code.
"It felt like we were dealing in smuggled diamonds.
"I couldn't believe the condition of it - almost like the day it was made. I can't imagine a better copy can be found.
"Only 500 copies of the true first impression hardback were printed, 300 of which were sent to libraries and schools.
"This new discovery is scarcer than the previous find, which was a former Staffordshire library book. It deserves to fetch much more."
The book will go under the hammer in Etwall, Derbyshire, alongside 11 more Harry Potter first editions - one signed by J K Rowling .
Mr Spencer, associate director at Hansons, added: "Due to amazing coverage across the UK and as far afield as Japan and Korea, we were flooding with emails and enquiries from people who thought they might have a first edition.
"At one point I was receiving 50 emails a day."