Those whose familiarity with Oliver Sacks extends only to his vivid book titles — “The Island of the Color­blind,” “An Anthropologist on Mars,” “The Man Who Mistook His Wife for a Hat” — may picture his writing as a gallery of grotesques, a parade of the exotically impaired. Sacks, a practicing neurologist, does specialize in case studies of highly unusual patients. But even as he entertains and diverts with his dramatic tales, Sacks has always been up to something else: he is gently educating us about the frailties and flaws — and the strengths and capacities — of “normal” people, those whose afflictions are of the most ordinary sort. You may never have confused your spouse for an item of outerwear, but have you ever failed to recognize the face of an acquaintance? Fumbled for a word that eluded your grasp? Read a sentence three times and still didn’t get it?