Venetian Navigators, By Andrea di Robilant

From the hieroglyphics of Aztec Mexico to the red stripe of London's Central Line, maps are merely idealised representations of the world; some maps have served as instruments of intimidation and control. The first surveys of the Scottish Highlands, notoriously, were done to facilitate the crushing of rebel clans in the wake of the Jacobite uprising of 1745. Two centuries later, Nazi map-makers redrew Europe's frontiers in the shadow of the swastika, with an emphasis on "Jew free" areas of conquest.

Venetian Navigators, by the Italian-born historian and journalist Andrea di Robilant, is an account of 14th-century map-mania and the Italian navigators who charted apparently new-found lands in the North Atlantic. When the Venetian brothers Niccolò and Antonio Zen first began to explore these lands in the 1380s, Europe did not yet have its national maps. Instead, emblazoned with monarchical insignia or overtly patriotic, they had a purely symbolic function.

Nevertheless, in their determination to map terra incognita, the Zen brothers claimed to have stumbled on a series of islands in Scottish and Scandinavian waters with the strange names of Frislanda, Estland, Drogeo, Icaria and Islanda. Whether these islands really existed was of little concern to them: European cartographers could use the "discoveries" as a source for a new mapping of the globe.

John Dee, the astrologer and adviser to Elizabeth I, later seized on the "Zenian islands" to press the case for a British Empire in northern seas. An English translation of the fratelli Zen's explorations (first published in Italian in Venice in 1588) had impressed Dee with its possibilities of maritime expansion in territories as yet "unspoiled" by Spanish and Portuguese settlers. Elizabethan merchants were keen to traffic in seal skins, cod liver oil and seal blubber.

Wretchedly, however, in 1835 the Zen story was exposed by a Danish admiral as a "tissue of lies" and the brothers were consigned to oblivion. Were their voyages a hoax? Di Robilant thinks not. Doggedly, he sets out to rescue the Zens from their ill repute as fablemongers and carto-illiterate fantasists. What the brothers "discovered" in the North Sea, the author believes, were today's Orkney Islands, the Faroes, Shetland, Iceland and (possibly) Greenland.

In this hybrid of travelogue and history, di Robilant muses on questions of Venetian cartography and navigation as he follows in the brothers' footsteps across Scandinavia, the northerly outreaches of Scotland and, above all, Venice. The result is an absorbing work that vividly recreates post-medieval Venice as a glittering mercantile outpost of the Adriatic. As scions of Venetian merchants, the Zen were proud of their ancestry and very much a part of the city's maritime glory.

If Venetian Navigators has a fault, it lies in the clichés that occasionally cling like grime to the prose. People arrive in "the nick of time", wars are "crippling", and seas are "choppy". On the whole, though, this is a diverting and delightful book, with numerous Renaissance-era nautical maps to amuse us along the way. Those of us with poor visual-spatial skills, however, may find it just as easy to read these maps upside down, as they make little sense to the modern eye.

Ian Thomson's 'The Dead Yard: Tales of Modern Jamaica' won the Royal Society of Literature Ondaatje Prize