Let me start with a confession: I wanted this book to be lousy. You see, one of my oldest dreams has been to do Borneo -- trek through it and write a rip-roaring adventure/travel book about the experience. Now here was some upstart stealing my tropical thunder. I was prepared to commit critical mayhem.

But a funny thing happened on the way to the pillory. I found that I had on my hands a small masterpiece, a book so intelligent, informative and amusing as to disarm jealousy. And so I've come to praise "Into the Heart of Borneo," not to bury it.

The upstart in question is Britisher Redmond O'Hanlon, himself -- oh, gall and wormwood -- a book reviewer and naturalist. In 1983 he and a friend, the poet James Fenton, set out for the mountain fastness of northwest Borneo, specifically the interior of Sarawak, to see if they could locate any living specimens of the Borneo two-horned rhinoceros, Didermocerus sumatrensis harrissoni.

This is a noble mission, comparable to Peter Matthiessen's quest for the snow leopard that inspired his book of the same name. Yet the two works could hardly be more different. Whereas Matthiessen, the existential trekker, is all earnestness and sobriety, O'Hanlon, the larking naturalist, is mostly exhilaration.

Some of the contrast may be attributable to climate and terrain. Negotiating raw passes in the oxygen-poor Himalayan heights, one might well take the austere, meditative tone that infuses "The Snow Leopard." But tramping through the fecund hothouse of Borneo, the slithering ground of leeches and rat-sized roaches and myriad other forms of excessive growth, one would be hard pressed to keep a sense of the ridiculous in check. At any rate -- and fortunately -- O'Hanlon didn't even try.

For one thing, he exercises an uncanny ear for fractured diction, and a native guide named Leon, who had made the English language his own, kept feeding the writer splendid material. The combination makes for passages like this:

" 'Redmon,' he said sotto voce, 'I hopes you and Jams not go with hotel girls?'

" 'I haven't seen any hotel girls.'

" 'They on top floor. Very naughties.'

" 'Do you go with hotel girls?'

" 'No, Redmon,' said Leon, with great seriousness, 'there is new diseases here. Your spear it rots. You go to hospital, they look at your spear, you take medicine. We have a word for this diseases. I not know it English. We Iban, in our language, we call it syphilis.' "

O'Hanlon also has an eye for what Evan Connell calls "the luminous detail" -- that extra bit of observation or fact that quickens a manuscript. Here are he and Fenton each reading part of a dismantled paperback copy of "Les Miserables": "In the shade cast along the edges of the flood-level river-bed by the forest giants, we read quietly, wedged comfortably into the rounded rocks, our arms and shirts and trousers and feet a sweaty feeding ground of salted moisture for two clouds of iridescent butterflies."

Another set piece depicts the ever-reading Fenton's routine reaction as the canoe in which he is riding approaches rapids. "With a second or two to spare, James would shut his book, mark his place in it with a twig, slip it neatly under an edge of the tarpaulin, place his left buttock upon it, shut his eyes, get drenched, open his eyes, squeeze the water from his beard with his right hand, retrieve his book and carry on reading." Though virtually every detail in that passage is luminous, for me it's the twig that has the real candle power.

I don't mean to imply that O'Hanlon lacks gravity. He devotes ample attention to flora and fauna (and tracks down every rhino rumor, to no avail). He also adds a footnote to ecohistory. The great 19th-century English biologist Alfred Russell Wallace identified a line dividing Borneo and New Guinea, on either side of which the wild species were "startlingly different" from those on the other. Today it is clear, O'Hanlon explains, that the Wallace line marks the "subterranean join where the Australasian tectonic plate drifted into the Indo-Asian landmass." But the reader seldom has to turn more than a page before encountering another outburst of O'Hanlon's high spirits -- an invaluable asset for anyone obliged to endure so grueling a trip.

The men who guided O'Hanlon and Fenton belong to a tribe that formerly practiced headhunting; this, as well as other cues, inspires O'Hanlon to devote several pages to a subject that must intrigue anyone drawn to Borneo: the legendary wild man. On the basis of his friendship with and reliance on his guides, O'Hanlon agrees with Wallace's conclusions that all races of Homo sapiens have the same brain capacity, that the differences between civilized and uncivilized people are slight.

"Into the Heart of Borneo" is a book to be savored slowly, but I lacked the willpower. Having gobbled it up, I want more from O'Hanlon, who is reportedly afloat on the Amazon. As for my own adventuring -- well, there is always Sumatra. By Dennis Drabelle; the reviewer is a Washington writer and amateur naturalist.