Gallic gloire in a North African sandpit

The Sahara, which derives its name from the Arabian word sahra , meaning desert or wilderness, is the world's largest dryland. It covers a similar area to the US, including Alaska. Even the eastern portion, confusingly called the Libyan or Western Desert (even though most of it is not in Libya and it is western only in the context of Egypt) has an area and shape comparable to the whole of the Indian subcontinent. Occupying about one-third of the African continent, the Sahara stretches from the Sahel bushlands of West Africa to the Mediterranean. It is not the driest desert on earth - a distinction belonging to the Atacama of Chile - but it is still very dry, with many areas receiving less than 2.5cm of rainfall a year. It is its size and its extremes of temperature that are daunting. Nineteenth-century geographers, who had not necessarily had personal experience of its nature, painted a dire picture of what was plainly to them an immense abomination. Elisée Reclus reported that even the flea itself would not venture into its wastes, that travellers might suffer from the râgle , a kind of brain fever that causes delirium, and warned that "when the wind blows hard, the traveller's body is beaten by grains of sand, which penetrate even through his clothes and prick like needles". Warnings of this type may well have weighed upon Lord Salisbury, who at the height of the colonial partitioning of Africa generously said the Gallic cockerel could have the Sahara and scratch it as he would. The British had their eyes on more productive and appealing territories, including the Nile Valley.

For a long time not only did the Sahara itself seem repellent, but many of its inhabitants also sought to repel the Europeans. The Tuareg harried the French, whereas the Senussi gave the British and the Italians a torrid time in Libya and Egypt. However, after a long struggle the French succeeded in colonising their part of the Sahara, and this in turn enabled travellers to explore and, remarkably, to develop extraordinary enthusiasm for it. Fergus Fleming tells a story relating to the French conquest of the Sahara, while Marq de Villiers and Sheila Hirtle attempt to explain the fascination that the desert can exercise on those who explore it.

The Sword and the Cross concerns two friends and contemporaries who were devoted to the gloire of France and made the French conquest of the Sahara possible: Charles de Foucauld and Henri Laperrine. They could not have been more different in character or method. Foucauld, as an orphaned teenager, lost his religion and his innocence, squandered a large inheritance, became immensely corpulent and gluttonous, took a mistress and joined the French army for adventure. Laperrine, two years Foucauld's junior, was tall, thin, celibate, badly groomed, a disciplinarian and a serious soldier.

However, in 1889, after travels in Morocco and Palestine, Foucauld, in one of the most remarkable transformations of all time, changed from a debauched officer into a Trappist monk. He gave his remaining wealth to his sister and resigned his army commission. In due course, he found even Trappism too comfortable, and so set up an isolated hermitage in the Saharan mountains, hoping to convert the tribesmen. He was notably unsuccessful as an evangelist, but remained a committed imperialist and provided the French authorities with vital intelligence. He was killed at Tamanrasset by dissident Senussi in 1916, "a victim of his charity and his apostolic zeal".

Laperrine's role was very different. Ever the soldier, he formed a camel corps for the long-range pursuit of the Tuareg and made heroic and difficult journeys, sometimes guided by Foucauld. He was on the Western Front when he heard of Foucauld's demise and asked for an immediate transfer to North Africa. Between 1917 and 1919 he set about avenging his friend's death. In 1920 he was killed in a flying accident during a sandstorm, and was buried next to Foucauld.

Fleming has crafted a great story around the lives of two remarkable individuals: Foucauld, famous, a voluminous writer, revered as a saint and much written about, and Laperrine, elusive, largely forgotten, who wrote little.

De Villiers and Hirtle have produced a very different sort of book, described in the dust jacket as "a glittering geographic tour of the remarkable history, peoples, climate, creatures, sights and sounds of the largest and most austere desert on earth". It covers a great deal of ground, but it is not based on the same intensity of research as Fleming's book, and neither is it, inevitably, such a gripping story. The structure is also rather staccato, lurching breathlessly from one topic to another.

Nonetheless they succeed in transmitting the many surprises of the Sahara - its immense reservoirs of underground water, its enormous dunes, its rich archaeology, its resilient fauna and flora, and much besides. It is "a place full of nature, endlessly persistent, magically inventive, astonishing in all its particulars". One of the "surprises" that emerges is that the Sahara has not always been a desert. It has a verdant past, when large lakes formed, underground water resources accumulated, Cape buffalo roamed, big rivers meandered through swampy floodplains and people lived there in profusion. The last time this happened was about 9,000 years ago.

The change to its present state was sudden, and by 3,600 years ago, the Sahara was in a state of parched aridity. This surprise is well explained.

Less happy is the discussion of the Sahara's majestic sand dunes. Here the lack of serious research becomes evident as the authors overplay the rapidity with which dunes come and go. They argue that bare plains develop dunes over 30m high in just a few weeks, and massive dunes can turn into a flat sand sheet a month later. Perhaps they do, but it is not the normal mode of behaviour of most dunes.

Rather surer are the discussions of great dust storms. The Sahara produces enormous quantities of dust that are lifted to great heights in turbid skies that extend far out of the desert - to Barbados in the west and Baghdad in the east, to the Alps in the north and to Abidjan in the south.

The storms carry pathogens and pesticides that have health implications for humans and corals, but they also dump nutrients that feed the organisms that produce toxic red tides as far away as the Gulf of Mexico.

Intending travellers to the Sahara, if not put off by a spate of murders in southern Algeria, will find much to enthuse them in this book. They will learn of intriguing mysteries that will repay deeper study.

Andrew Goudie is professor of geography, University of Oxford.