''Throwim Way Leg'' is a book of wonder and excitement, struggle and sadness, a love letter to Papua New Guinea and Irian Jaya. In ''New Guinea Pidgin,'' Tim Flannery explains, 'throwim way leg means to go on a journey. It describes the action of thrusting out your leg to take the first step of what can be a long march...' Here he invites us to share in his breathtaking adventures, as he meets skilled hunters and befriends a shaman, climbs mountains never before scaled by Europeans, discovers new species and, deep in the jungle, stumbles across the giant bones of extinct marsupials. He also writes movingly about the fate of indigenous people when their intricate cultures collide with mining companies and the high-tech modern world.