Linked by the events of Bernard Knox's remarkable life, the twenty-five chapters of ''Essays Ancient and Modern'' cover subjects ranging from Hesiod, Homer, and Thucydides to Auden, Forster, and the Spanish Civil War. With a masterful eye for the telling detail, Knox continually reminds us that we share the present with antiquity's living past. A soldier in Italy finds a battered book in the rubble of a bombed-out firehouse-- and opens it to read Virgil's denunciation of war. An illiterate Greek bard composes a garbled Homeric song to celebrate the recent heroism of local partisans. A traveler heading north from modern Athens must choose between the Sacred Way-- or the NATO Road. Whether the subject is the role of women in ancient Athens or the novelists of modern Italy, the wit and erudition of Bernard Knox never fail to instruct and delight. Now in paperback, ''Essays Ancient and Modern'' takes it place alongside the distinguished essays of Knox's ''Word and Action'', a book whose title brings together, in the words of Anthony Hecht, ''the double strand of his admirable career''.