Isabella: She-Wolf of France, Queen of England by Alison Weir

Isabella has been vilified. History nails her as a harridan and harlot: the nemesis of a weak homosexual king; the adulterous lover of a rapacious Earl with whom she tyrannously ruled the country. When her son came of age, Isabella ended up under house arrest for 28 years, apparently raving mad.

Until now there has been no published full-length biography of this infamous queen. In her weighty Isabella, She-Wolf of France, Queen of England the popular historian Alison Weir attempts an act of rehabilitation. Consulting numerous contemporary sources, Weir boasts a select bibliography covering 29 pages. Her book is jam-packed with details, from the colour of Isabella's carpets (black) to the cost of Brie cheese she sent a friend (27s 6d). No one could accuse Weir of slacking on research.

Born around 1295, Isabella was the daughter of the powerful French King, Philip IV. At 12, she was married off as part of the solution to an on-going problem regarding English territories in France. Yet the child-bride, who was educated (she could read) and beautiful (a plump blonde), joined a kingdom nearly bankrupted by continual war with Scotland. The nobility had "grown resentful under the iron fist" of Edward I and with the new king vied to regain its lost privileges.

Edward II, 24 and handsome, preferred digging ditches and trimming hedges to more military pursuits. If that baffled the barons, they were outraged by his other tastes. Edward flaunted his homosexuality and indulged his favourite, boyhood companion and lover Piers Gaveston. Poor Isabella was soon calling herself "the most wretched of wives", complaining to Daddy she had nothing to live on since the income Edward promised her had not materialised, Gaveston was wearing her dowry jewels and her husband was giving her bed a wide berth.

Focusing on personalities rather than policies, Weir shows Isabella's early marriage years as marked by the persistent standoff between the king and the barons determined to oust Gaveston. The political implications of Edward's fecklessness, however, were far more serious than his sexual predilections. Failure to address properly the Scottish issue led eventually to the humiliating defeat at Bannockburn, scuppering England's hopes of subjecting Scotland.

Isabella loathed Gaveston but supported Edward, finding attacks on the royal prerogative (the Lord Ordainers wanted to curb the king) more offensive than her partner's amour fou. Following Gaveston's murder in 1312, Isabella could finally act like queen. She was pregnant and was overspending around £10,000 a year. But Edward's reign continued to lurch from disaster to disaster. And then came the dangerous new favourite, Sir Hugh le Despenser. Isabella called him "an intruder" in her marriage who tried "to dishonour her by every possible means". Again she was sidelined and when a French war loomed in 1324, Isabella's estates were seized and her four children taken from her.

Such treatment, Weir suggests, fuelled a "desire for vengeance" in the formerly dutiful wife. She now acted like a woman scorned. Edward foolishly sent her across the Channel, along with his heir, to mediate. Isabella began her affair with Roger Mortimer, and vowed to stay in France until her rival was banished. When she did return she was leading a successful invasion.

While Weir finds "much to like" about Isabella, readers may part company with her. The detested Despenser experienced the standard traitor's death - but not before being castrated publicly. Since Isabella was present and castration wasn't a common punishment, it's fair to assume there was some sexual triumphing going on. Weir argues that Isabella knew nothing of the regicidal plot and questions whether Edward died, as every schoolboy knows, from having a red-hot poker thrust up his anus. Edward's lover was savagely punished for sodomy, so why not the king?

Throwing the blame still further from Isabella, Weir favours the conspiracy theory that Edward wasn't murdered at all and finished life as a hermit in Italy. She details an escape route which sounds as complicated as it does implausible. It would mean, according to Weir, that Isabella has been wrongly condemned for a crime that never was.

This meticulous no-nonsense biography presents a fascinating story complete with puzzles. Did Isabella give birth to Mortimer's illegitimate children? If Edward wasn't killed, who was buried in his place? But Weir wants to render a "realistic portrait" and when a biographer can only speculate on her subject's thoughts it's a tough call.

Isabella was raised to be a mother of kings. That was her defining role and determined her behaviour far more than personal sensibilities. It's hard to believe therefore, as Weir does, Isabella's protestations that she was "in great trouble of heart" over Edward's death. Isabella was an ace dissembler and clearly no sheep in wolf's clothing.