The dangerous combination of deep-rooted, perhaps unconscious, prejudice and lack of knowledge lies at the heart of Anne Edwards' Throne of Gold. (Skin colour is important as a motif to her: only European women need dark glasses and parasols to protect themselves from the sun in Saudi Arabia, for example). Edwards sets out to tell the story of the present Aga Khan, his father and grandfather: the leaders of the Ismaili sect of Muslims. In its scope, it has promise. All the ingredients of exotica are present, including the court of Queen Victoria.

Edwards has written about figures like Vivien Leigh and Ronald Reagan; perhaps she had a Hollywood script in mind when she undertook this book. It is not to be relied on for accuracy: Sind, a major province of Pakistan, is placed in Afghanistan; the birth dates of the main characters are inaccurate, and so on.

The single impression that emerges of the senior Aga Khan is of a dark-skinned male forever lusting after white-skinned women. His story as one of the triumvirate who changed Muslim history in South Asia and gave the community a sense of identity is not explored. Born in 1877, at about the same time as the other two figures, the Aga Khan' s financial and political acumen combined with Iqbal's poetic vision and Jinnah's leadership to form a major force in Indian politics.

Similarly, the central focus of the present Aga Khan's life is missed. His rural development programmes in north Pakistan are the one glimmer of hope in converting entire villages from poverty into self-respecting self-sufficiency. His hospital in Karachi is the best run in Pakistan; his patronage of Islamic architecture based in Harvard-MIT has generated global interest. Most important, he has become a bridge between east and west, between Muslims and non-Muslims, tradition and modernity.

Yet Edwards panders to the fantasy of the Aga Khan in popular mythology: as an oriental despot of fabulous wealth, forever linked to fast horses and fast women. She is a political innocent. Along the way she insults - I suspect unwittingly - not only the Ismaili community and the larger Muslim one but also revered national figures like Gandhi and Jinnah.

We come away with little idea of the spiritual needs and sociological compulsions of the Ismailis. Why do they respond to the person of the Aga Khan? What role does he fulfil for them, both spiritually and materially, and how does their faith in him survive in this age of secularism? Reading Edwards' book raises the question of how we are to treat belief and tradition in an age dominated by a cynical and irreverent media.

The best way to deal with "offensive" books by those who are sensitive to them is either to ignore them or to refute the arguments through debate. Burning them will simply raise their sales. The Ismailis appear to have learnt this cardinal lesson. I am not an Ismaili, but I believe that attacking a spiritual figure like the Aga Khan or a royal one like the Prince of Wales is an unfair sport. They cannot respond. As for the idea of free speech and expression, in our world of violent ethnic conflict, it needs to be taught with respect for others, for ordinary human feelings, for the diversity of plural societies and mixed communities.

Akbar Ahmed, Fellow of Selwyn College Cambridge, was chief consultant for the BBC series "Living Islam"