HEADACHE RELIEF <i> By Alan M. Rapoport MD and Fred D. Sheftell MD (Simon & Schuster: $18.95) </i>

This is not a book for hypochondriacs. The authors have produced a comprehensive look at a medical problem that affects more than 90% of the U.S. population at one time or another each year. Most of us gobble down an over-the-counter remedy--64 million aspirin tablets a day taken only for headaches, plus other, non-aspirin pills. In 1987, we spent more than $2.1 billion for these pain relievers. But 20% of all Americans suffer headaches so intolerable that they are forced to seek medical help. If they are unlucky, they see doctors who either don’t take them seriously or who put them on medications that may be addictive, and often don’t relieve the pain.

Or, if they are lucky, they go to people like the authors, founding directors of The New England Center for Headache. Rapoport and Sheftell believe that headaches are triggered by a biological mechanism, “apparently one the headache sufferer is born with,” an unhappy revelation for those who don’t believe in predeterminism.

To conquer the pain, no matter its source, the authors prefer about anything to a pill. They believe that headaches can often best be treated with exercise, biofeedback techniques and diet. They do prescribe medication where necessary, but only in the smallest practicable dosages.

The book’s opening list of various kinds of headaches (cluster headaches, for example, are worse than migraines--so painful that sufferers often bash their heads against a wall) and their causes (from brain tumors to “tension”) is as sobering as viewing an auto wreck. Its only consolation is that most of us can say, “There but for the grace of . . . “ to most of the forms of headaches.

There is one bit of humor in that list, certainly unintentional, and the only laugh in the book: a brief discussion of “benign orgasmic cephalgia”--sex headaches. The straight-faced medical advice about when to take the problem to an emergency room for evaluation could certainly inspire a screamingly funny movie scene.