Thomas Becket: Warrior, Priest, Rebel, Victim, By John Guy

Sub-titles are usually where publishers try to hype up a book, or give its subject a headline-making spin. So Viking should be congratulated for their Ronseal-like directness – "what it says on the tin" – in tagging John Guy's biography of Thomas Becket as "a story retold". For the tale of the 12th-century Archbishop of Canterbury and the king who appointed him, but came to rue the day and saw him murdered in his cathedral, is even on the primary-school curriculum.

It is therefore hugely to Guy's credit that he manages to make his book such a compelling read. There is almost no evidence produced here that is new – a few marginal notes he has uncovered in a book once owned by Becket hardly counts as ground-breaking. Moreover, this Cambridge academic and occasional broadcaster sensibly avoids taking extreme positions to inject a frisson of controversy.

On the essential question that has been asked so many times over 900 years – was Becket a saint? – he offers a marvellously measured verdict. The archbishop may have been a showman, and certainly had not adhered to high principles throughout his life. But he was honourable, charismatic and had right on his side in his clash with the overbearing, bullying, vain Henry II over the relative rights and privileges of Church and State.

The joy of this book lies in the telling. Here Guy has form. His take on another well-covered life, of Mary, Queen of Scots in My Heart is My Own, won the 2004 Whitbread Biography Prize. He knows how to take the familiar and shape it into a narrative that both improves our historical knowledge and is entertainingly astute, and in places positively moving.

So Guy weaves together the various biographies/hagiographies written of Becket in the years immediately after his brutal death – and coloured by that event – with the bigger political picture in Europe. In the process, he teases out the nuances. Again, this is well-trodden territory. He explores the homoerotic charge to the once-close, then hostile relationship between Becket and Henry – the vassal who bit back. He questions Becket's own sexuality: Becket was a relatively late recruit to the chaste life of priest, entering the service of a previous Archbishop of Canterbury, Theobald, as a layman. And he points out – as Henry did – that as Chancellor for seven years, Becket had been happy enough to go along with the king's dictatorial ways and rough justice.

In places, Guy does risk drifting into psychobabble in his analysis of the two personalities that dominate this book. The worst excess is when he describes Becket as "always anxious and insecure by temperament" and attributes that to being "closest to his mother as a child", but generally he steers a safe course in such treacherous waters.

A little more on Becket's spirituality might have added an extra something, especially since his life is in one sense a build-up to his martyrdom. Details of his fasting, self-flagellation and sleeping on floors - instead of in the plump beds provided by his hosts - deal only with the outward sings of inner grace. Did he really believe himself chosen by God? Or did he simply find himself holding the poisoned chalice of Church-State competition and decide he had no alternative but to drink? The problem Guy confronts in all such matters, as he candidly admits, is that "given the often intractable nature of sources written 900 years ago, some things can never be proved one way or the other". The biographer's aspiration, of course, is always to have the last word. Here Guy accepts that he won't.

For those rusty on the machinations of kings, queens, dukes, minor royals, popes and bishops in the 12th century, this is your chance to give your knowledge a polish that is pleasant as a gentle massage. Guy clearly and succinctly guides readers through the various disputes over the succession to the English crown among the descendants of William the Conqueror, the overspill from such clashes into France, and he provides vivid portraits of a cast of familiar names, not least of Eleanor of Aquitaine.

Most of us tend to start out young with high ideals and then see them chipped away by economic necessity, disappointment, earthly career ambitions and weariness. So by framing Becket's story as the tale of one who embarks on adult life lazy, listless and without real ambition, and then steadily grows more principled, disciplined and self-sacrificing, Guy gives his biography an added message for our times. It will, for example, make for essential reading for the candidates for the post of Archbishop of Canterbury. How often have recent incumbents started out in the role with a reputation for standing up for what they believe, and left diminished by the compromises they have made? Not very Becket-like.

Peter Stanford's 'The Extra Mile: a 21st-century pilgrimage' is published by Continuum