Pick of the paperbacks; The Third Wave, by Alvin Toffler. New York: Bantam Books. 537 pp. $3.95.

In his first major book since the prize-winning ''Future Shock,'' Alvin Toffler says, the future already has begun. Or, put another way, the present has long since begun to grind to a halt. And when was the beginning of the end? Probably Aug. 8, 1960, the day an Exxon executive decided to stop paying some of the taxes his firm had been charged by oil-exporting countries. This was the move that prompted the founding of OPEC (the Organization of Petroleum Exporting Countries), and everybody who uses money knows what has happened to civilization since then.

But the inflation, inequities, and inefficiencies of today are also the steppingstones to tomorrow. And a promising tomorrow it should be, Toffler maintains -- at least as livable as the present and potentially more decent and democratic. ''The Third Wave'' focuses on where change will take us. It is an important book, as much for its synthesis of history as for its optimistic glimpse of the future.