YEATS SAID THAT THE Bodleian Library in Oxford was the most beautiful library in the world; in fact, the Bodleian consists of a number of beautiful medieval, Jacobean and 18th century buildings, but also the prewar New Bodleian, described by Jan Morris as looking like a municipal swimming bath, and often called the ugliest building in Oxford. It is here, in an important redevelopment, that the library will be opening its doors to the public, to display some of its little-known but truly heart-stopping collection of manuscripts and treasures.

Julian (Toby) Blackwell (of the bookshops), whose Oxford branch has long been intimately entwined with university life, has made a gift of £5 million towards a display centre for the treasures of the Bodleian. And what treasures they are, from a Gutenberg Bible, one of only eight surviving, to Tolkien drawings, to a first edition of Over the Rainbow.

There are utterly unexpected things, such as the Marconi Archive, which includes the telegrams the company received when the Titanic was sinking, and music scores, including Gustav Holst's original score for The Planet Suite. There are more than 10,000 medieval manuscripts, and important collections of Islamic, Chinese, Indian, Japanese and Hebraic works. The collections includes the papers of six prime ministers - though not of Margaret Thatcher, who fell out with the university in 1985 after she was refused an honorary degree - and a quarter of all the surviving Magna Carta documents.

Magna Carta will undoubtedly be one of the star attractions of the new display. It is the earliest constitutional document of Britain, the agreement with King John in 1215 that led to the reforms that came to be seen as the basis of parliamentary democracy and a model for so much of the world, including the United States of America.

I have been lucky enough to see many of the greatest literary treasures of the Bodleian, including the original manuscript of Kafka's The Castle, ending dramatically with the unfinished sentence “aber sie sagte...” (“but she said ...”) which Kafka's editor, Max Brod, eliminated from the printed work; the Venerable Bede's Acts of the Apostles; the earliest known copy of La Chanson de Roland, and the Shakespeare first folio that the Bodleian, as a library of legal deposit, received in 1623; for 400 years it has received by law a copy of every book published every year.

This first folio has a romantic history: when the Stationer's Company sent it to Oxford, it was bound by William Wildgoose, a local stationer, and placed in Duke Humfrey's Library, where it was attached by a chain to the shelves. But when the third folio, which contained seven more plays, was published in 1663, Oxford sold its first folio as superseded. In the early part of the 20th century a young Oxford graduate, Gladwynn Turbutt, appeared at the library with a copy of the first folio, which he had found in his family's library. Because of the marks on the brown calf binding and the papers with which the binding was stuffed by Wildgoose, it was recognised as Oxford's own. There was a scramble to find the money to buy it back: first folios - there are about 290 in the world - had even then become the subject of what has been called “bardolatory”, but of all these, the Bodleian's first folio is recognised as the most important because of its authenticated first 40 years at Oxford and its original binding. Incidentally, from the wear and tear of the pages, it can be deduced that between 1623 and 1666, Romeo and Juliet was the students' favourite play, with the balcony scene more worn than the rest.

When these treasures go on show in the new Blackwell Hall - some time after 2010 - I predict that the Johnson Collection of Ephemera will be very popular. This collection was made between the late 1920s and the mid-Fifties by John Johnson, a printer and papyrologist: he described it as “a little museum of common printed things to illustrate at one and the same time our social life and the development of printing”. Categories include advertisements, artists, authors, bookbinders and publishers. Johnson also collected tickets, menus and greetings cards. The result is powerfully evocative, because so much of it is an unmediated and naive glimpse of a certain time.

The Harding Collection of Printed Music came to the Bodleian in 1974 from Chicago in 900 packing cases. It was collected by a British-born Chicagoan ragtime pianist, Walton N.H. Harding. He had never seen the Bodleian, but he left the library his astonishing and comprehensive collection of songbooks, song sheets, folk and instrumental music and music hall ballads. He also collected opera scores, poetry, drama and travel literature. So important is this collection in the history of modern music, that the Bodleian is digitising it - a huge task - to satisfy the worldwide interest. In fact the Bodleian has already digitised more than a million items from its collections, because it is clearly impossible to give unlimited access to everything in the library.

The Bodleian, founded in 1602, actually has its origins a lot earlier, in the bequest in the first half of the 15th century of 250 manuscripts to Oxford by Duke Humfrey, youngest brother of Henry V. No library has a wider human reach than the Bodleian because of these unbroken years of collecting, deposit and gifts. The result is a particular magic: artefacts that movingly demonstrate the hopes, aspirations and speculations of mankind. In a few years the full extent of this magic will be gloriously evident.

Justin Cartwright is the author of This Secret Garden: Oxford Revisited (Bloomsbury)

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