For critics and scholars, the greatest rewards are to be gained in bestowing attention on authors whose stock is already high. Why, then, should a writer look toward one of the forgotten? And how to select someone to elevate? This is the task that Hua Hsu, an associate professor of English at Vassar, sets for himself in his smart new book, “A Floating Chinaman: Fantasy and Failure Across the Pacific.” His chosen subject is the mostly unknown H. T. Tsiang (1899-1971), a Chinese immigrant who “created some of the most ambitious and, at times, bizarrely self-aware works of modern American literature.”