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Baudolino
by Umberto Eco
522pp, Secker & Warburg, £18

Italo Calvino described a "deep-rooted tradition in Italian literature... the notion of the literary work as a map of the world and the knowable, of writing driven on by a thirst for knowledge that may in turns be theological, speculative, magical, encyclopaedic..." He was talking about Dante the visionary and Galileo the cosmographer, but he himself, and Umberto Eco, also work from the same impulses.

Eco's new novel, set during the sack of Constantinople in 1204, derives from Boccaccio its form of stories told during a crisis, but has things in common also with the fabulating fantasy of Calvino's Imaginary Cities. It is the life-history of Baudolino, a self-confessed liar, told to the Byzantine historian Niketas Choniates. It is fiction - Eco's, Baudolino's, tall-storytellers' of the ancient world - woven into the history of the fourth crusade.

At the centre of the novel is a brilliant conceit about how the human mind makes up its world. It is an examination of the deep need for explanatory stories - myths, fables, chronicles, family traditions, science - and works in codes and layers that resemble the medieval methods of biblical interpretation as much as modern semiotics. It turns, like the Christian religion, on questions of paternity and the presence of the divine spirit in matter such as the blood and wine of the Eucharist.

Baudolino lives a Freudian family romance. His father Gagliaudo is an Italian peasant, but he tells the story of how he became an adopted, favoured son of the Holy Roman Emperor, Frederick Barbarossa. He falls in love - in the classical situation of chaste courtly love and distant desire - with Barbarossa's wife, Beatrice of Burgundy, a kind of passion that thrives on fantasy and invention. He writes poems for a poet who loves an imaginary woman he has never seen. He marries, almost incidentally, a 15-year-old girl who dies in childbirth and gives birth to a real, dead, monster.

Baudolino's personal family romance at another level is reflected in the endless theological struggles of the time about the exact relation of the Christian Trinity, Father, Son and Holy Spirit, the exact nature of the flesh of the Virgin and the Son, the nature of the presence of the dead God in the blood and wine of the sacrament. This is Eco's field and the debates are both detailed, obscurantist, comic and brilliantly baffling.

The family romance extends itself into the romance of travel, the romance of legend, and the romance of desire. Baudolino is a forger of fictions - his love poems resemble Cyrano, he makes up a letter from the legendary Prester John, ruler of a fabulous faraway land, and devises imaginary maps of the known or speculative world. He gets involved in the business of fake relics - several dried heads of John the Baptist, nails of the True Cross, and finally the lost Grasal, or Grail, the Cup of the Last Supper, which Baudolino invents by using the worn wooden bowl from which his dying (real) father drinks.

"Credo quia impossibile" (I believe because it is impossible), Tertullian said about the Christian story. Theologians have also argued that our desire for God is a proof of his existence. Baudolino and a group of travelling companions - the poet in love with an unseen woman, a Jewish rabbi looking for the lost tribes, a pair of quarrelling scientists disputing the existence of a vacuum, a nasty priest called Zosimos - call Prester John and his fantastic land into being simply by setting out in search of it, following their forged maps and their trumped-up letter.

They meet imaginary beasts and monsters - giants and creatures with eyes in their breasts, one-footed hopping skiapods and beautiful hypatias, half-virgin, half-goat - creatures out of the fantastic voyages of Mandeville, turned solid in this tale. The creatures themselves turn out to be in constant dispute about the substance of the Son and the Father, the real and the fantastic mirroring each other.

The idea of the vacuum is also part of this complex and multi-layered trope. "Nature abhors a vacuum" and things rush into the emptiness of created vacua, both in the flask and in the mind. If God did not exist it would have been necessary to invent him.

Eco is a student of the varieties of inventiveness with which we fill up the spaces of our knowledge and our experience, from air to corpuscles, from daydreams to poems, from lies to myths, from forgeries to monsters. The death of Frederick Barbarossa is presented as a classic "sealed room" mystery, from the modern detective story, in which the emperor might have been killed by any number of ingenious devices, including a vacuum-making machine. We are unmade too in imaginative ways.

Here, in this set-piece, is a clue to why this novel is not, as a novel, anywhere near as successful as The Name of the Rose. That very satisfactory tale was written, Eco once said, because he wanted to murder a monk. It is held together by being, at one of its levels, a sustained parody of a Sherlock Holmes investigation. It carries its formidable learning lightly, using it both as clues and as red herrings, as set-pieces and as rich background, interesting all sorts of readers quite incidentally in medieval theology and Aristotle. The narrative discipline, and the drive, was provided by the Conan Doyle artifice, and the Agatha Christie-like series of consecutive murders.

Baudolino has no such anchor in contemporary or near-contemporary forms. Nor is it tightly connected to ancient forms, the Decameron or medieval travel narratives. It is a modern would-be rollicking tale, sprawling over time and place, fact and fantasy, and - apart, significantly, from its opening - it has no real voice of its own.

Its opening is Baudolino's attempt to write in garbled mixed languages on a stolen parchment, which he has scraped clean of the Imperial Chancellor's official historic documents, which obtrude from time to time in blackletter where the scraping has been unsuccessful. This combination of palimpsest and mixed colloquial Latin, Italian, German and other languages does give Baudolino a substantiality and a knottiness which he loses when he simply starts talking. He talks a kind of rushed slapstick - he sounds like a popular historical romance of, perhaps, the 1950s - and his author's control of the pace and tension of his narrative is much less secure than it was in The Name of the Rose.

The marvels of Prester John's realm are not quite Rider Haggard, nor yet a parody of Rider Haggard, nor yet a parody of Mandeville. They have no voice of their own and remain flat and two-dimensional, not fully imagined, nor yet distanced, hinted and layered. If the palimpsest had persisted there would have been more texture, even though it would have been irritating. It is a peculiar kind of novel where the difficult ideas are more interesting than the swashbuckling, or the sex, or the death, or the gruesome objects. It is a paradox that Eco's most readable tale is also his least satisfactory - an insubstantial body, informed by the ghost of a brilliant idea.

· AS Byatt's novel The Whistling Woman is published by Chatto

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