Marsden Hartley is one of our best painters, but not long ago he could have been described as an enigmatic presence in the history of this country’s art. Not that he was unrecognized. Quite the contrary, but he was known for a fraction of his work. We knew him as a pioneer modernist, a member of the Stieglitz circle, an expatriate who went to Europe early in this century and spent much of his life abroad. We knew that he returned to America toward the end of his life to find new subject matter in the harsh landscape and vigorous people of his native New England. Or something like that. Our awareness of Hartley’s development as a painter and our sense of him as an individual remained vague. Surprisingly, no major study of his life and work appeared until 1980, when the Whitney Museum of American Art mounted a full-scale retrospective and published Barbara Haskell’s biographical catalogue.