I often think Play was my best work,"" states Kenneth Koch in ""The Artist,"" a hilarious parody of artistic creation and/or confusion. Word-play, dream-play, ego-play--all the elements of the lawless imagination, particularly as we experienced it in adolescence, are present in Koch's ebullient and often fantastic world. Far removed from the light verse of Ogden Nash and Phyllis McGinley, or the satiric bite of Cummings, Koch's humor is closest to the surrealist sweetness of Apollinaire or Max Jacobs. It attempts, as he once remarked, an ""incomprehensible excitement,"" recreating ""an excitement I had felt,"" as well as producing comic metaphors that suggest the screwball universe of the Marx Brothers or the Keystone Kops. The language is inventive, exclamatory, and buoyant, always both thoroughly impressionistic and colloquial. But in its deepest and most singular moments, it ends on a note of mysterious sadness or absurdity, as in ""A Poem of the Forty-Eight States,"" where the youthful, leaping syntax unexpectedly registers an elegiac tone: "". . . I used to have so much fun during the summer, cooking and kidding and having myself a good time,/ I like Pennsylvania too, we could have a lot of fun there,/ You and I will go there when Kenneth is dead."" Koch is in his mid-forties, so it is arguable whether the style he has forged will be able to withstand the pressures of age. Happily, The Pleasures of Peace shows no diminution of his spirited talent.