Six o'clock- end of a trying day -- dinner ahead -- the companionship of a woman- and a drink. Preferably, according to Mr. DeVoto (who is a man of strong opinions) a Martini, America's gift to the gods. Or failing that, a good rye. No rum. No sweet drinks. No whimsical accompaniments, gags or gadgets. These are his convictions and he makes no bones about them in this brief, sprightly, sometimes humorous, sometimes serious dissertation on the fine art of drinking. A plus sale item- a week-end gift.