When the gods decide to transform Marina Warner into another creature, they will probably pick the hummingbird. Dart and hover is her characteristic manoeuvre, an iridescent shimmy from subject to subject. Keeping up can be exhausting, but in this typically agile meditation, she rewardingly charts shifting images of the imagination itself.

In Ovid's great poem, Metamorphosis, humans migrate into myriad new forms, whether as punishment or refuge. Many of the transformations are a source of terror, but there are far worse fates than mutability. Worse, surely, to be left stock-still, like Ovid's Niobe turned to ever-weeping stone.

Warner's endlessly protean writing, by contrast, conducts a rolling investigation engulfing everything from Joan of Arc to fairy tales, from civic monuments to intimate lullaby. This new study, based on an Oxford lecture series, opens with Ovid and wonders how ideas of metamorphosis have themselves altered, tweaked by colonial explorations, by science and new technologies.

Warner's gift is for inspired juxtaposition. Fantastic Metamorphoses, Other Worlds is less an argument than a magic lantern show that takes us from Caribbean creation myths all the way to the personal demons of Philip Pullman's His Dark Materials trilogy. Partly because Warner's readings are so palpable and vivid, because we leap so dizzyingly from Frankenstein to Lewis Carroll's fairy fantasies, we may forget that her book is about migrating souls rather than transformed bodies.

How does metamorphosis create an idea of self? Ovid's characters retain their essence even when decanted into new forms - a laurel, a spider, a nightingale. He paints a landscape of intense individuality skipping into unexpected shapes. To medieval theologians, the notion of bodies wrenched from their humanity was the devil's work. Dante's damned souls are blotted out in transformation, all personhood cruelly erased.

Warner suggests that metamorphosis might work as natural law, rather than sinful aberration, as she finds seventeenth-century Dutch entomologist Maria Merian in the West Indies, marvelling as lumpy caterpillars become lace-winged butterflies. Selves can emerge from transformation in implausible guise, and Warner typically darts ahead to watch Kafka's hapless Gregor Samsa embodying his inner beetle or Nabokov's luscious Lolita entering drab adulthood. This last progression may disappoint needy Humbert Humbert, but Nabokov, himself a lepidopterist, knows better than to second-guess what nestles in the pupa.

In her underrated novel, The Leto Bundle, Warner allied a mutating classical myth to fury at the refusal of asylum and, by extension, a historical mistrust of dialogue with the other. That theme recurs as she discusses Western explorations of the New World transforming both the observers and the observed. They plundered gold but ideas, teasing fantasies, were stowaways back home. She relates the nineteenth century's swoon into the supernatural to the spirit beliefs of new colonised subjects, in an idiom she calls 'Imperial Gothic'. Here, Brazilian myths of bodies stumbling on with out their souls - zombies - provide chilling images for the age of slavery. Journeying spirits increasingly travel inward, as novels by James Hogg and Robert Louis Stevenson tremble with the idea that we may contain shadowy doubles, our minds housing the stranger within.

The book tingles with illustrations: from the infant Helen of Troy hatching from an egg 'in the manner of a soubrette from a cake' in an ancient Greek sculpture to a ghoulish phantasmagoria. A delicious section slips into Bosch's Garden of Earthly Delights, his panel of pleasure flanked by scenes of Eden and Hell. In this 'cornucopia of metamorphosis', sleek, naked humans get frisky with giant fruits - slumped over strawberries, bobbing at blackberries, balancing currants on their heads. Bosch is familiar for his tormenting monsters, itchy hybrids of animal and mineral, but Warner celebrates his 'eloquence in communicating pleasure - blissful, paradisal, serene festivity'. Did Bosch draw on giddy myths new-harvested from the Americas? The garden's fruity delights may enact a moment of innocence before colonial ambitions darkened and, with them, the stories brought back from new territories, marinating in imperialism's fear and anger.

Academia doesn't always know how to regard Warner, writing like an angel and interested in everything. When she gave the Reith Lectures, I heard an Oxbridge academic sniff: 'She's not what I'd call an intellectual.' But what makes Fantastic Metamorphoses remarkable is its dashing investigation of imagination. Hence her insistence that Western discovery was a two-way trade in fantasy that 'offered extraordinary possibilities for thinking differently'.