Tourists who wander bewildered around Oxford asking: "Where's the university?" might well also ask: "Where's the cathedral?. For Oxford's cathedral is not at the heart of the city, as in Winchester, Gloucester or Salisbury, but inside Christ Church, looking rather like any other college chapel.

In a sumptuous new book called Cathedral (Constable, £30), Jon Cannon lists it as England's smallest cathedral. He tells how the Anglo-Saxon minster church was transformed by the medieval cult of St Frideswide into a major Augustinian priory by 12th-century aristocrats. After Frideswide's death in 727, the runaway virgin escaping King Algar's forced marriage, who owned a little settlement beside the oxen ford over the Thames, was acclaimed a saint. As the author says: "No account of Frideswide's life survives before the early 12th century: she may be an invention."

The priory spent most of the 14th century locked in disputes with the university and citizens of Oxford, and would have been reduced to rubble in Henry VIII's Reformation had it not been for the sudden death of Cardinal Wolsey. Three-quarters of it survived - only to be dominated by Wolsey's unfinished cloister. St Frideswide's was lucky - its rival as Oxford's cathedral, Oseney Abbey, lies beneath the railway station, with its famous bell now in Tom Tower.

Cannon's book is a labour of love, providing a fascinating view of the strange world that created England's cathedrals, and bringing their architecture vividly to life.

Jon Cannon will be at Blackwell's, Broad Street, Oxford, on November 15 for a book signing.