As Malcolm Cowley says in his introduction, the first edition of Leaves of Grass ''might be called the buried masterpiece of American writing'', for it exhibits ''Whitman at his best, Whitman at his freshest in vision and boldest in language, Whitman transformed by a new experience.'' Mr Cowley has taken the first edition from its narrow circulation among scholars, faithfully edited it, added his own introduction and Whitman''s original introduction (which never appeared in any other edition during Whitman''s life), and returned it to the common readership to whom the great poet really speaks.