When Jan Morris sat down to write her acclaimed fictional travel account Hav, she had a whole city to invent. The "south-facing, self-contained" place had no history, no geography and no citizens beyond those in her imagination. Employing her witty, evocative prose to full effect, she walks the reader past noisy pavement cafés and into crumbling public buildings, to the shores of huge lagoons and down the paths of beautiful parks. All the time, we're meeting the weird as well as the wonderfully ordinary; hoteliers, intellectuals, musicians, politicians. There are even the notes of Marco Polo and other bygone visitors to enrich our sensory enjoyment.

It all works  the fake anecdotes, the invented colonisers, the eccentric customs  because Hav is famously cut off from the world. And because we have never been there.

The Guianas are not dissimilar on both counts. Bar the Mongolian steppes or the Kalahari, there can be few inhabited places on the globe that remain quite so secluded. The Guianas  literally, the Lands of Many Waters  stand as South America's minnows, small and slightly sodden. The three dwarves squash head-to-toe along a swampy coastline of 900 miles. Inland, towards Brazil in one direction and Venezuela in the other, they boast little but bug-strewn forest and gargantuan rivers. Little wonder, as John Gimlette wryly observes, that these isolated lands have "never been truly possessed".

All of which leaves the author a nearly blank page when he shows up in Georgetown "uncertain what I'd find". His experiences do not disappoint. The next 300-odd pages provide a rollicking, witty and informed account of what crawls out of the Guianas' jungles into the author's path. Armed with a rucksack and a keen sense of the absurd, Gimlette drives, flies and floats through Guyana (formerly British Guiana), Suriname (formerly Dutch Guiana) and French Guiana (or Guyane Française, and still very much French).

He tarries longest in Guyana, which occupies the bulk of the book. After the one-escalator capital of Georgetown (a city "so physically transparent yet so mentally opaque"), he makes the obligatory trip out to Jonestown where more than 900 members of a US cult killed themselves in a mass suicide pact. From there, he travels into the forested hinterland on the trail of dead gold-diggers and long-gone plantation owners. In Suriname, the jungle paths turn up communities of Amerindians and the descendants of runaway slaves. French Guiana provides a rather dispiriting conclusion, all French bureaucratic métros sweating in the heat and eerie penal colonies. Even the story of Henri Charrière (alias Papillon, the country's famous escapee) turns out to be "magnificently improbable".

Remarkably, little seems to get Gimlette down. The few writers to have made it to the Guianas are almost universally downbeat. Evelyn Waugh hated every moment. "Destructive and predatory" was his verdict. VS Naipaul described the inhabitants as "deceitful and sullen". Only Walter Raleigh had anything good to say, but then he was high on the idea of finding El Dorado. Gimlette relishes the fact that "the only creatures [he] ever saw were things trying to eat [him]". He is an old-school traveller, very British, very cheery.

A barrister by trade, the author has an uncanny ability to nail down his characters with a few well-chosen words. Take Dango, the hunter, who is "barrel-chested, splay-toed, uncomfortable in the open". Or Colin Edwards, "half Welsh, a bit Basque, wild in his dreams and loud in reproof". Morris couldn't have invented a more exotic cast  which all makes you wonder if the cameo sketches aren't just a fraction over-dramatised. But the book's real strength lies not in the living, but the dead. This is fortunate. The Guianas have a habit of curtailing human life. In the early days, if the Amerindians didn't get you, malaria or dysentery certainly would, and recent decades have added cocaine and Kalashnikovs. That leaves Gimlette, an avid historian, with a problem. The Guianas' past has largely returned to nature. With dogged determination, he is forever visiting forts or Dutch planters' palaces and discovering them "ploughed back into the mud". His vivid descriptions of Guiana's "X-rated Garden of Eden" help to cover some of the gaps. But however "drenchingly fecund" the forests or "mirror-black" the rivers, the reality is difficult to hide: away from the arboreal and aqueous, there's simply not much to see.

Fortunately, Gimlette brings history to life. He artfully merges assiduous research with a storyteller's gift, presenting the reader with a catalogue of loveable rogues, inveterate explorers, eccentric naturalists and idealistic romantics. These stories provide more than just a good yarn. They offer a door into the darker aspects of the Guianas' past  and thereby a window on to much of the present. The most prominent historical headache is slavery, which, he found, "every strand of Guianese life somehow led back to". But colonialism, violence, immigration, race prejudice and indigence get a look in as well. A few hardy souls might just be inspired to follow him to South America's wild coast.

Oliver Balch's Viva South America! A Journey Through a Restless Continent is published by Faber.