Rare and valuable books that have sat unopened on the shelves of Blickling Hall library for more than 100 years are being catalogued for the first time.

Specialist rare book librarian John Gandy has spent more than two years researching just a fraction of the 12,500-strong collection and estimates it will take a further eight years to finish the job.

The books date from the early 11th Century to the 19th Century and some individual tomes have been valued in the highs six figures.

The collection, which is the largest and most important in the whole of the National Trust, includes the 900-year-old Dialogue of Pope Gregory the Great and the third atlas ever printed, published in 1482. It also features three Latin bibles printed before 1500 and first editions of three Jane Austen novels.

The library was bequeathed to Blickling's owner the Earl of Buckinghamshire in 1745 by his cousin and avid book collector Sir Richard Ellis of Lincolnshire, who feared his beloved collection would be split up and sold off after his death by his widow's next husband.

When the family left Blickling around 1900, the house was rented by a succession of tenants and gradually knowledge of the contents of the great library were lost. The National Trust began restoration work at Blickling in 1960 but has only recently been able to focus on the library.

Now Mr Gandy is methodically working thorough the collection, which is comparable to the great foundation collections at the universities of Oxford and Cambridge, to establish the purpose and provenance of each book, including who owned it, the cost and why they were reading it.

He said: 'We are rediscovering what the library is, why is it here and who was Richard Ellis, Even that knowledge had gone. The intention now is to make that knowledge available.'

The ancient books must be cleaned regularly to ensure their survival for years to come. A team of specially trained National Association of Decorative and Fine Arts Societies (NADFAS) volunteers work one afternoon every week to dust the volumes and check their condition.

Books found with beetle or mould problems are cleaned in a fumigation cabinet by a member of the house staff trained in conservation techniques and more seriously damaged articles are sent away to a specialist

It takes volunteers seven years to work from one end of the library to the other and, once they have finished, it is time to start again.

Catalogued library contents can be searched on-line at copac.ac.uk or nationaltrustcollections.org.uk.