A fresh, rich, urgent report on the largest wildlife expedition to visit the Himalayas and its stalking ground, the more than 20,000-foot deep Arun Valley in far eastern Nepal--which harbors five climatic zones (tropical to alpine), more species of birds than the continental U.S., and quite possibly that mysterious half-man, half-ape, the yeti. But this Shangri-la is worse than endangered, ornithologist and expedition co-leader Cronin warns, it's threatened with ecological collapse. Cronin begins by telling about the wide-eyed discovery of a salamander--""the first salamander ever found in Nepal."" He recounts how he and the other Sahibs naively set forth equipped with all sorts of paraphernalia and bent on setting the pace for their porters--only to be humbled at the first, exhausting day's close. ""The porters arrived later, in the cool hours of the evening, joking, intolerably pleasant after their chang [the local beer]. There was, it appeared, more mind than muscle in walking."" Cronin's account, then, is a double-edged story of discovery and recognition, speckled with natural-history insights. A lowland sal forest is surprisingly open and spacious and empty--the result of annual burning to promote new grassgrowth for the overabundant sheep. Farther up a forest seems to be intact, but many animal species are missing--the outcome of selective cutting (for lumber and fodder) on the natural, sustaining vegetation. A pattern thus emerges of the stages ""in the gradual progression from a natural environment to a man-dominated one""; and the mountain people perceive the danger, Cronin discovers, while the authorities close their minds (""In Nepal,"" he was told, ""conservation has to do with tigers and rhinos""). Much of Cronin's chronicle, however, concerns the wildlife he did find--he's at his most beguiling on the ""resource-based polygyny"" of the orange-rumped honeyguide (whose several mates are attracted by the bees wax he controls)--and the footprints outside his tent that strongly suggest the yeti's presence. (Why, he asks, so much resistance to what is not ecologically implausible?) This is manifestly no Snow Leopard. But Cronin clearly knows and loves what he's about, and his plain, serviceable prose (marred only by some querulousness) makes it matter to the reader.