"It's a bit like asking how high is up," Eric Holzenberg, director of New York's 125-year-old Grolier Club private library and exhibition centre, said yesterday.
"My stock answer is a rare book has some combination of age, scarcity and desirability.
"For example, bibles can be old but nobody wants them, and some modern fine print [limited edition] books can become rare as soon as they are published."
David Pearson, rare books librarian at the University of London, said he believed every early book could be rare and unique, depending on how it was bound and whether it was annotated.
"The Folger Library in Washington has 80 copies of the first edition of The Collected Works of Shakespeare, published in 1623.
"Every one has its own individual characteristics because of the way it has been bound, the notes written inside and the leaves which may have been torn out.
"To have all 80 is enormously useful to scholars who want to study them as a group."
The men are in Dunedin for the annual Australian and New Zealand rare books school, being hosted this year by the University of Otago.
The five-day school, which has attracted a student from the United States as well as 22 others from various parts of Australia and New Zealand, is held here every four years.
Mr Pearson is teaching about the pedigrees of books and their provenance, while Mr Holzenberg and university special collections librarian Dr Donald Kerr are co-tutoring a course on exhibiting rare books.
They said the rarest books in the world were those which were known to have existed - usually because they were listed in publishers' catalogues - but had since disappeared.
The most expensive rare books were those such as the first collection of Shakespeare's plays which could sell for as much as 3 million ($NZ7.8 million).
Mr Holzenberg has been collecting books since he was a teenager, specialising in architecture, before qualifying as a librarian.
Mr Pearson said he "grew up surrounded by books" and began his career looking after rare books in cathedral libraries in Britain.
Both said it was the thrill of finding a previously undiscovered rare book which had kept them interested for more than 20 years.
Both said they liked to fossick in second-hand book shops and book sales, and had developed the knack of being able to swiftly spot a potential treasure.
The arrival of the Internet had made rare books accessible to collectors worldwide, including themselves, they said. Mr Holzenberg said his best Internet find was a 1912 Sears-Robuck catalogue.
On a whim, he decided to enter his own unusual surname into the e-bay site and found a catalogue which contained that name.
It turned out to be his grandfather, who had purchased a house through the company and written a testimonial.
Mr Pearson, who collects books which have paper owner labels, said he had bought many books via the Internet, including one from New Zealand.
Both hoped to be able to explore Dunedin's second-hand book shops, once the intensive book school finished on Friday, and said if they got the opportunity, they were sure they would find something of interest.
"One thing you can be sure of is that unusual and unexpected treasures still remain to be uncovered," Mr Pearson said.