“It has to be said, there are no points to be won from liking Hesse nowadays.” This rueful assessment of the novelist Hermann Hesse, quoted in the opening pages of Gunnar Decker's new biography, “Hesse: The Wanderer and His Shadow” (Harvard), appeared in an obituary in 1962; but it could just as well have been pronounced yesterday, or a hundred years ago. Ever since he published his first novel, in 1904, Hesse has been one of those odd writers who manage to be at the same time canonical—in 1946, he won the Nobel Prize in Literature—and almost perpetually unfashionable among critics. The great German modernists who were his contemporaries mostly disdained him: “A little man,” according to the poet Gottfried Benn; “He displays the foibles of a greater writer than he actually is,” the novelist Robert Musil said. In America today, Hesse is usually regarded by highbrows as a writer for adolescents. Liking him is a good sign at age fifteen, a bad one by age twenty.