SARAH GRISTWOOD'S previous books, Arbella and Perdita, have been revealing, well-researched biographies about relatively little-known historical figures. With her third, however, she has chosen to profile an extensively studied British monarch, Elizabeth I, and one of her most pored-over affairs, her relationship with Robert Dudley, Earl of Leicester (the subject of the recent TV mini-series starring Helen Mirren).

Gristwood has no new information or material to offer, so it's a risky enterprise, but she is a sharp and perceptive biographer and her qualities help her produce an accessible, thought-provoking portrait. The Tudor period will always be fascinating, it seems, for the way power was largely brokered not only through lengthy diplomatic treaties or complex military campaigns, but through sexual attraction, secret plots, untimely deaths and suspicious circumstances.

Elizabeth's route to the throne has been detailed many times before by writers such as Phillippa Gregory and David Starkey, and Mary, Queen of Scots biographers Alison Weir, John Guy and Jane Dunn.

Gristwood is more sympathetic to Elizabeth than others have been; she acknowledges that Elizabeth's regular and flagrant breaches of protocol, as regards Dudley, did her damage in the early days of her reign but she also emphasises the loneliness of her position.

Unlike Mary Queen of Scots, Mary of Guise or Catherine de Medici, other women in positions of power, she was the only one not subject to "the spiritual power of Rome".

She did not have to bow down before the Pope, but nor could she then call on him for assistance.

This isolation is tempered by the constant presence of Dudley, a friend from her adolescence and one of her closest advisors.

He is, of course, long rumoured to have been her lover, even the father of a child she secretly had.

Gristwood doesn't sidestep the issue of whether the Virgin Queen was a fabrication designed to unite a country riven by religious conflict, but her own evidence would seem to go against her conclusion.

Gristwood believes the threat of pregnancy would have prevented Elizabeth from risking penetrative sex.

She does not admit the possibility that people, even queens with much to lose, take huge risks for love, and it is apparent that Elizabeth loved Dudley.

The question, though, is not whether Elizabeth and Dudley ever actually had sex, but whether their relationship is important to history.

Gristwood argues this point much more convincingly when discussing the influence of "favourites", those invariably sexual partners more common to kings. That influence demonstrated the extent of a king's power, and the same, it would appear, was true of queens.

Thus we see Queen Anne's fawning letters to her favourite Sarah Churchill demonstrating just how much a weak queen could debase herself; and Catherine the Great of Russia granting her lover Potemkin the kind of military power no ordinary commander could have dreamt of.

Elizabeth may have adored Robert Dudley but she was never fawning. Although she gave him power, there were always more powerful men such as Cecil or Walsingham around her.

Gristwood accuses Elizabeth of "using Dudley as a stalking horse" when she wanted to avoid marriage; of keeping him close yet never committing to him; of appearing to take his advice then prevaricating.

Perhaps the most important decision she made where his influence can really be detected, though, is in the execution of Mary, Queen of Scots. It was Dudley who finally persuaded her - his Protestant politics being much more radical than Elizabeth's and his fear of a Catholic head of state more pronounced.

Without his counsel on this matter, Mary might have lived.

James I certainly thought so, shunning Dudley's illegitimate son when he came to the throne on Elizabeth's death, blaming Dudley for his mother's beheading.

While Gristwood is saying nothing very new here, her focus on the Dudley-Elizabeth relationship emphasises not just his role in decision-making, but the role of the "favourite" in general.

A study on those lines alone might have made for a more intriguing read altogether, and if Gristwood had managed to resist the lure of the Tudors, with all their blood and pageantry, she might truly have found something new to tell us.