Bruce Chatwin's travel book ''In Patagonia'' (1977) earned wide praise and distinguished awards. In it he yielded only occasionally to the urge for novelistic invention that overcame him this time. He set out to write a biography of a Brazilian slaver in Dahomey, but he was arrested in Cotinou and something happened - he doesn't say what - that made him decide not to go back for more research. Thus he wrote this ''work of the imagination.'' How can we object? None of us has seen these things.