Cases containing the works, part of an exhibition open to the public on the development of English literature, were forced open. The theft was discovered on Thursday but the last confirmed sighting of the two manuscripts and five books was the previous Friday, when the exhibition closed.
A university spokesman said: "The... library is not a hideaway library. We're part of the heritage of the region and we put on displays. There is always a risk in putting things on show but nothing on this scale has ever happened before."
It was unclear how the works were taken from the building without anyone noticing, but university staff were said to be distressed at the "irreplaceable loss".
The works would be "virtually impossible to sell on the legitimate market". The three most valuable items were a manuscript of an English translation of the New Testament from the late 14th or early 15th century, a manuscript including the fragment of a Chaucer poem from about the same period, and a copy of the first printed edition of Shakespeare's collected works, known as the First Folio edition, from 1623.
The Shakespeare was particularly significant, as it was bought new by Bishop Cosin, when he was Bishop of Durham, and was the only First Folio edition to have stayed in the original collection in Durham. Among the other treasures were a 1566 edition of works by the 10th-century writer Aelfric and an edition of the Anglo-Saxon epic Beowulf printed in 1812.
A police spokesman said the library did not seem to have been broken into and it appeared the theft had taken place while it was open.
Members of the public, as well as students and academics, had access to the room containing the stolen works which were among 50 exhibits charting English literature from the Middle Ages to the 20th century. The cases were covered in draped covers to protect the books from light.
Durham is not the first university to be hit by theft. An Oxford don, Dr Simon Hughes, was jailed for two years in 1995 for stealing and selling 78 old books and manuscripts from Oxford and London universities.Reuse content