This is a little clamshell of a building, its curving white masonry sides giving away nothing of its function. It stands like a bastion at the windy hilltop entrance to the 1960s campus of Lancaster University, alone in what will be a meadow of waving grass. Richard MacCormac, its architect, has attempted to distil the essence of John Ruskin, the great - and quite possibly mad - 19th century commentator on art, architecture, economics and the human condition, into this metaphorical ivory tower. It is a tall order.

Officially opened on May 9, 1998, this is an example of a very rare breed indeed: a fine new building funded by Britain's Heritage Lottery Fund. Norman Foster's American Air Museum at Duxford is the other main example. There will be few more - the Fund has decided this is not really its bag, and has accordingly rejected Daniel Libeskind's proposed Imperial War Museum in Manchester, though it may well finance a new National Library for Women in London. The justification in Lancaster, as at Duxford, was the lure of a ready-to-roll project with a valuable historic collection and its matching funding all set up. And it is gratifyingly small, which always helps when you are doling out semi-public funds. £3.2m, of which £2.3m comes from the Lottery, is a bagatelle.

You can read the Ruskin library any number of ways - as a ship, an island, a church (MacCormac has previously built a chapel at Fitzwilliam College, Cambridge, which is similar in form), a jewel casket, a reliquary. Just as Ruskin retreated into splendid isolation at Brantwood on Lake Coniston, so his eponymous new library stands aloof not only from the rest of the campus, but even from the new university library behind. That is also by the practice of MacCormac Jamieson Prichard, but is stylistically wholly different. What makes the Ruskin Library seem like a jettisoned escape pod from the British Library is its central concept of being a building within a building.