Everyone who plays pool knows that it is ''mostly mental, '' but the conventional wisdom about the mental game is about as accurate as the idea that the earth is flat. Until now, no one with any expertise on how the human mind works has bothered to write about pool. In ''Pleasures of Small Motions,'' Bob Fancher, a psychotherapist and pool columnist, breaks new ground by applying good science to the mental game of billiards.This book does for pool what Timothy Gallwey's bestselling ''The Inner Game'' books did for golf and tennis. Fancher explains how the conscious and unconscious mind work together, prescribes drills to help players improve, advises on mastering emotion and developing rhythm, explains the difference between concentration and focus, and gives invaluable insight on competitive play.