Its bleak title, Final Exit, suggests a steely existential tract or perhaps the last Cold War thriller. But Derek Humphry's unlikely best-seller hit a deeper and darker nerve in the culture: the growing public anxiety over the right to die. The book—plotless, short on action, packed with charts—amounts to a do-it-yourself guide to doing yourself in. His hardcover alternative to Dr. Jack Kervorkian's suicide machines has sold over 500,000 copies, thanks to its lists of lethal dosages of common prescription drugs, hints on how to quicken the process (a plastic bag over the head) and legal advice for would-be accomplices. It has also landed the author at ground zero in the ethical and legal war over euthanasia, which will only intensify as our ability to prolong life outpaces our ability to make it worth living.

Humphry, 61, argues that the terminally ill who are mentally competent have the right to choose death. "We're talking about dignity and pride," he says. "My book may help sort out the complexities." The British ex-journalist became involved with euthanasia in 1975, when he helped his wife, dying of breast cancer, kill herself. In 1978 he moved to the U.S. and, with second wife Ann Wickett Humphry, founded the Hemlock Society (motto: Good life, good death). They divorced in 1990 after Ann, too, contracted breast cancer; in October she also committed suicide.

Perhaps Final Exit's success should startle no one. Last year there were nearly 31,000 recorded suicides in the U.S. Americans' ambivalence about euthanasia was evident last month when Washington State voters rejected a measure to allow doctors to help terminal patients end their lives. But Humphry is buoyed that some 690,000 were in favor. "The right to choose to die," he said, will be "the great debate of the 1990s."