FORGET REINCARNATION. What if, instead of merely being born again in a brandnew body, the eternal human soul could pass from one body to another while the bodies` owners were still alive? If such a thing were possible, then during the lifetime of one body, several different souls might take turns inhabiting it. Enter Ian Adams, who has based the plot of his latest novel, Becoming Tania (McClelland & Stewart, 304 pages, $26.95 cloth), on the implications of this idea. Nicolas Quintana, Adams`s Argentinian hero, is a former army officer -- now a pacifist -who spends his time reading occult books and giving Tarot readings. Then into his careful life blows a woman who calls herself Tania Gutierrez Bauer, after the late guerrilla heroine of Latin America who was Che Guevara,-, lover. Tania`s effect on Nicolas`s life is dramatic: when he is picked up and tortured by the secret police, his psychic powers blossom until he is able to reconstruct the tragic story of Che and his ragged band of freedom fighters. Almost no one is what he or she seems in this stunning novel, which succeeds in bringing to life a legend from Latin America`s past.