Of the ‘legal deposit’ libraries in the UK and Ireland, I have visited the British Library, the National Library of Wales in Aberystwyth and Trinity College Library in Dublin. Only the last named looks like a library to a visitor, with rooms, floor to high ceilings, lined with shelves packed with books. In the other two no books are visible. You fill in a form, sit at a table and wait, and the requested volume is brought to you. But safely stored somewhere in the inner recesses of these great buildings, what wonders there are. ‘Cotton Vitellius A.xv’ may not ring bells widely. Similarly, one might say, ‘Cotton Nero A.x’, and ‘Cotton Vitellius A.vi’. They do, however, linger in my memory. The codes identify manuscripts, the first Beowulf, the second Sir Gawain and the Green Knight – wonders of Anglo-Saxon and early English poetry – and the third is a twelfth-century manuscript of the De excidio et conquestu Britanniae by Gildas. They are in the British Library, where they continue to be classified in this way because the original owner, Sir Robert Bruce Cotton (1571–1631), designed a system whereby his collection was placed on shelves surmounted by busts of Roman emperors: ‘Otho A.xii’ should be The Battle of Maldon, but in 1731 it was destroyed in a fire (fortunately, having been copied beforehand), along with a dozen or so others. Beowulf, though singed, survived.

The National Library of Wales, founded in 1909, is a relatively late addition to the world list. It is spectacularly housed in a classically proportioned white building high on ...