The exciting haul - which was found behind a hidden cupboard - includes a 1538 edition of letters by the famous Roman philosopher and statesman Marcus Cicero.
As well as books dating back to the 1530s, the collection includes 17th and 18th century volumes on surgery, witchcraft and exploration.
It also features a rare 1827 edition - one of only 50 copies - of John Milton's Paradise Lost, with illustrations designed and engraved by the visionary artist John Martin.
It's believed the secret cupboard, which was behind an old plan chest, might not have been opened in more than 30 years - and the discovery of the fascinating historical cache by archivist Neil Dickson has created huge excitement.
Neil, who was working his way through the archives when he made his find, said: "I couldn't believe my eyes when I saw the cupboard which obviously hadn't been opened for 20, 30 years or longer.
"I was absolutely stunned when I realised what was inside.
"I have never seen such a collection of books in one place and certainly not under these circumstances.
"I am sure the find will be of enormous interest."
Inverclyde Council's education committee convenor Councillor Terry Loughran is keen to encourage as many people as possible to view the historic items.
Councillor Loughran said: "This really is an incredible find and one that will be talked about in literary circles up and down the country and beyond.
"As well as books that are nearly 500 years old there is also a 19th century Hamnet edition of Shakespeare's plays, published in Greenock and edited by the then librarian of the Watt Library, Allan Park Paton.
"It is testament to the long traditions of academic excellence in Greenock that such a collection found its way to the Watt Library over the years and I look forward to finding out more about the circumstances and history of the find."
The collection will be the star attraction of an exhibition tomorrow which forms part of the BBC's Great British Story event at the McLean Museum. The event is free and will take place between noon and 4pm.