Frank Kingdon Ward's account of his 1924-25 expedition to the gorges of the Tsangpo, Tibet's principal river, surely rates as a classic. The gorges lie in Pemako, a rugged region in the extreme southeast of the country that uniquely combines what Ward calls the "hot hell" of a rainforest with the "cold hell" of the highest Himalayas. "Snakes and wild animals, giant stinging nettles and a myriad of biting and blood-sucking ticks, hornets, flies and leeches" abound. The only climatic constant is the perpetual deluge of rain, sleet or snow. Yet Ward made light of such inconveniences and somehow managed to make the place sound positively appealing. A plant hunter by trade and an explorer by necessity, he also wrote more delightfully than one has the right to expect of either profession. The expedition was his most dramatic and successful, and the book his finest. Long out of print, any new edition of it would be welcome.

But this sumptuous volume is much more than a reprint. Ward's discoveries, geographical as well as botanical, left "the riddle" of the gorges only partly resolved. Fired by his book, more recent travellers have endeavoured to follow the river further into the world's deepest, dankest gorge. Might there be another Niagara Falls lurking somewhere in these cloud-filled depths? Or, clinging to their vertiginous scarps, other botanical gems that, like the wine-red lily ( Lilium paradoxum ), had eluded Ward's attention?

With new contributions from a horticulturalist, a Tibetologist, an expedition leader and a geologist, this book carries the story forward to its final solution in 1998. More than 200 colour photographs complement Ward's descriptions of the plants and the terrain. And his text is now so encased in addenda (foreword, preface, two introductions, three sections of acknowledgements, four new introductory chapters and six new appendices) as to replicate in structure the calyx and corolla of the rhododendron blossoms, that are the region's pride and Ward's abiding passion.

Plant collecting makes for circuitous travel. During the flowering season, the collector must record and mark his quarry; several months later, he must return to search for seed. The colossal variations in altitude and rainfall between the snow-choked Himalayan passes and the fetid rainforests kept Ward on the move, but erratically and largely vertically. He was away for a year and secured "some of the most wonderful flowers ever brought out of the cold heart of Asia", especially primulas, rhododendrons and meconopsis. But without the geographical riddle of the Tsangpo, his narrative would be of interest solely to plantsmen and gardeners.

ÁñÁ«ÊÓƵ

The Tsangpo introduces a drama to which no lover of adventure can be indifferent. In 1907, the Swedish explorer Sven Hedin had traced its source with excruciating exactitude to a glacial rill in southwest Tibet. Thence it flows east across the Tibetan plateau for 1,600 miles, a well-behaved river collecting snow-melt feeders from the northern face of the Himalayas, which it parallels. It seems destined to loop round the eastern end of the range like the Mekong but suddenly, acquiring a mind of its own, it buckles, boils, charges the mountains and disappears, like bath water down a plug-hole, into a vortex of cataracts.

What became of it thereafter had exercised geographical minds for a century. The likelihood was that it emerged into Assam as the Dihang, the principal feeder of the Brahmaputra; but this was hard to prove. Attempts to push up the Dihang were frustrated by the terrain and attempts to push down the Tsangpo were impossible while Tibet was closed to foreigners. In the 1880s, India's map-makers endeavoured to get round this problem by despatching "pundits", Himalayan natives trained in basic surveying. They were to follow the Tsangpo as far as they could and then release into its foaming waters 500 pre-marked logs. Many thousands of feet below, where the Dihang emerged from the mountains, watchers would be waiting.

ÁñÁ«ÊÓƵ

Despite being robbed, beaten and sold into bondage, a pundit called Kinthup finally reached the gorges, cut his logs and discharged them. Unfortunately, by then several years had elapsed, his survey employer had left India, and he himself had been forgotten. Kinthup's Pooh sticks bobbed down to Assam unobserved. But his story, and especially its mention of a 300ft waterfall where the river disappeared into the mountains, attracted much attention. A 1913 expedition explored a section of the gorges and added further conjecture about a colossal waterfall. Ward then took up the challenge.

As well as drawing attention to the region's unique flora, Ward found more falls and at last got to grips with the horrific geography. Of the riddle of the river's Himalayan transit and its 10,000ft descent, there remained only a few miles of inaccessible gorge. In the 1990s, American teams equipped for abseiling finally closed the gap and discovered the mightiest of all Himalayan river waterfalls. The riddle then degenerated into the ridiculous as Chinese pioneers and scientists claimed to have sighted the falls first and to have surveyed the gorges in their entirety both from the ground and the air. Whatever the truth, Ward's account remains indisputably the best and this handsome book is a fitting memorial to perhaps the greatest of Himalayan plant hunters.

John Keay is the author of books on Himalayan exploration.